diff --git a/spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/web/servlet/WebMvcEndpointChildContextConfigurationTests.java b/spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/web/servlet/WebMvcEndpointChildContextConfigurationTests.java
index e4aa6e719f..3cc74b437f 100644
--- a/spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/web/servlet/WebMvcEndpointChildContextConfigurationTests.java
+++ b/spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/web/servlet/WebMvcEndpointChildContextConfigurationTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,7 +35,8 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 class WebMvcEndpointChildContextConfigurationTests {
 
-	private WebApplicationContextRunner contextRunner = new WebApplicationContextRunner();
+	private WebApplicationContextRunner contextRunner = new WebApplicationContextRunner()
+			.withAllowBeanDefinitionOverriding(true);
 
 	@Test
 	void contextShouldConfigureRequestContextFilter() {
diff --git a/spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnBeanTests.java b/spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnBeanTests.java
index b588ccfe8c..5e7c9b37c1 100644
--- a/spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnBeanTests.java
+++ b/spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnBeanTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -136,8 +136,8 @@ class ConditionalOnBeanTests {
 
 	@Test
 	void conditionEvaluationConsidersChangeInTypeWhenBeanIsOverridden() {
-		this.contextRunner.withUserConfiguration(OriginalDefinition.class, OverridingDefinition.class,
-				ConsumingConfiguration.class).run((context) -> {
+		this.contextRunner.withAllowBeanDefinitionOverriding(true).withUserConfiguration(OriginalDefinition.class,
+				OverridingDefinition.class, ConsumingConfiguration.class).run((context) -> {
 					assertThat(context).hasBean("testBean");
 					assertThat(context).hasSingleBean(Integer.class);
 					assertThat(context).doesNotHaveBean(ConsumingConfiguration.class);
diff --git a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java
index 0c116a86a8..ee7dfd87a7 100644
--- a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java
+++ b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,9 @@ import java.util.function.Supplier;
 
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.BeanDefinitionCustomizer;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.support.BeanNameGenerator;
+import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.boot.context.annotation.Configurations;
 import org.springframework.boot.context.annotation.UserConfigurations;
 import org.springframework.boot.test.context.FilteredClassLoader;
@@ -105,6 +107,8 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 
 	private final Supplier<C> contextFactory;
 
+	private final boolean allowBeanDefinitionOverriding;
+
 	private final List<ApplicationContextInitializer<? super C>> initializers;
 
 	private final TestPropertyValues environmentProperties;
@@ -124,13 +128,14 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @param contextFactory the factory used to create the actual context
 	 */
 	protected AbstractApplicationContextRunner(Supplier<C> contextFactory) {
-		this(contextFactory, Collections.emptyList(), TestPropertyValues.empty(), TestPropertyValues.empty(), null,
-				null, Collections.emptyList(), Collections.emptyList());
+		this(contextFactory, false, Collections.emptyList(), TestPropertyValues.empty(), TestPropertyValues.empty(),
+				null, null, Collections.emptyList(), Collections.emptyList());
 	}
 
 	/**
 	 * Create a new {@link AbstractApplicationContextRunner} instance.
 	 * @param contextFactory the factory used to create the actual context
+	 * @param allowBeanDefinitionOverriding whether bean definition overriding is allowed
 	 * @param initializers the initializers
 	 * @param environmentProperties the environment properties
 	 * @param systemProperties the system properties
@@ -139,7 +144,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @param beanRegistrations the bean registrations
 	 * @param configurations the configuration
 	 */
-	protected AbstractApplicationContextRunner(Supplier<C> contextFactory,
+	protected AbstractApplicationContextRunner(Supplier<C> contextFactory, boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super C>> initializers, TestPropertyValues environmentProperties,
 			TestPropertyValues systemProperties, ClassLoader classLoader, ApplicationContext parent,
 			List<BeanRegistration<?>> beanRegistrations, List<Configurations> configurations) {
@@ -149,6 +154,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		Assert.notNull(configurations, "Configurations must not be null");
 		Assert.notNull(initializers, "Initializers must not be null");
 		this.contextFactory = contextFactory;
+		this.allowBeanDefinitionOverriding = allowBeanDefinitionOverriding;
 		this.initializers = Collections.unmodifiableList(initializers);
 		this.environmentProperties = environmentProperties;
 		this.systemProperties = systemProperties;
@@ -159,15 +165,30 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	}
 
 	/**
-	 * Add a {@link ApplicationContextInitializer} to be called when the context is
-	 * created.
+	 * Specify if bean definition overriding, by registering a definition with the same
+	 * name as an existing definition, should be allowed.
+	 * @param allowBeanDefinitionOverriding if bean overriding is allowed
+	 * @return a new instance with the updated bean definition overriding policy
+	 * @since 2.3.0
+	 * @see DefaultListableBeanFactory#setAllowBeanDefinitionOverriding(boolean)
+	 */
+	public SELF withAllowBeanDefinitionOverriding(boolean allowBeanDefinitionOverriding) {
+		return newInstance(this.contextFactory, allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, this.classLoader, this.parent,
+				this.beanRegistrations, this.configurations);
+	}
+
+	/**
+	 * Add a {@link ApplicationContextInitializer} to be
+	 * calWebMvcEndpointChildContextConfigurationTestsled when the context is created.
 	 * @param initializer the initializer to add
 	 * @return a new instance with the updated initializers
 	 */
 	public SELF withInitializer(ApplicationContextInitializer<? super C> initializer) {
 		Assert.notNull(initializer, "Initializer must not be null");
-		return newInstance(this.contextFactory, add(this.initializers, initializer), this.environmentProperties,
-				this.systemProperties, this.classLoader, this.parent, this.beanRegistrations, this.configurations);
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, add(this.initializers, initializer),
+				this.environmentProperties, this.systemProperties, this.classLoader, this.parent,
+				this.beanRegistrations, this.configurations);
 	}
 
 	/**
@@ -181,8 +202,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @see #withSystemProperties(String...)
 	 */
 	public SELF withPropertyValues(String... pairs) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties.and(pairs),
-				this.systemProperties, this.classLoader, this.parent, this.beanRegistrations, this.configurations);
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties.and(pairs), this.systemProperties, this.classLoader, this.parent,
+				this.beanRegistrations, this.configurations);
 	}
 
 	/**
@@ -196,9 +218,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @see #withSystemProperties(String...)
 	 */
 	public SELF withSystemProperties(String... pairs) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties,
-				this.systemProperties.and(pairs), this.classLoader, this.parent, this.beanRegistrations,
-				this.configurations);
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties.and(pairs), this.classLoader, this.parent,
+				this.beanRegistrations, this.configurations);
 	}
 
 	/**
@@ -209,8 +231,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @see FilteredClassLoader
 	 */
 	public SELF withClassLoader(ClassLoader classLoader) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties, this.systemProperties,
-				classLoader, this.parent, this.beanRegistrations, this.configurations);
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, classLoader, this.parent, this.beanRegistrations,
+				this.configurations);
 	}
 
 	/**
@@ -220,8 +243,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @return a new instance with the updated parent
 	 */
 	public SELF withParent(ApplicationContext parent) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties, this.systemProperties,
-				this.classLoader, parent, this.beanRegistrations, this.configurations);
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, this.classLoader, parent, this.beanRegistrations,
+				this.configurations);
 	}
 
 	/**
@@ -256,8 +280,8 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @return a new instance with the updated bean
 	 */
 	public <T> SELF withBean(String name, Class<T> type, Object... constructorArgs) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties, this.systemProperties,
-				this.classLoader, this.parent,
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, this.classLoader, this.parent,
 				add(this.beanRegistrations, new BeanRegistration<>(name, type, constructorArgs)), this.configurations);
 	}
 
@@ -296,8 +320,8 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 */
 	public <T> SELF withBean(String name, Class<T> type, Supplier<T> supplier,
 			BeanDefinitionCustomizer... customizers) {
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties, this.systemProperties,
-				this.classLoader, this.parent,
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, this.classLoader, this.parent,
 				add(this.beanRegistrations, new BeanRegistration<>(name, type, supplier, customizers)),
 				this.configurations);
 	}
@@ -319,8 +343,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 */
 	public SELF withConfiguration(Configurations configurations) {
 		Assert.notNull(configurations, "Configurations must not be null");
-		return newInstance(this.contextFactory, this.initializers, this.environmentProperties, this.systemProperties,
-				this.classLoader, this.parent, this.beanRegistrations, add(this.configurations, configurations));
+		return newInstance(this.contextFactory, this.allowBeanDefinitionOverriding, this.initializers,
+				this.environmentProperties, this.systemProperties, this.classLoader, this.parent,
+				this.beanRegistrations, add(this.configurations, configurations));
 	}
 
 	/**
@@ -339,7 +364,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		return result;
 	}
 
-	protected abstract SELF newInstance(Supplier<C> contextFactory,
+	protected abstract SELF newInstance(Supplier<C> contextFactory, boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super C>> initializers, TestPropertyValues environmentProperties,
 			TestPropertyValues systemProperties, ClassLoader classLoader, ApplicationContext parent,
 			List<BeanRegistration<?>> beanRegistrations, List<Configurations> configurations);
@@ -388,6 +413,11 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 
 	private C createAndLoadContext() {
 		C context = this.contextFactory.get();
+		ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
+		if (beanFactory instanceof DefaultListableBeanFactory) {
+			((DefaultListableBeanFactory) beanFactory)
+					.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
+		}
 		try {
 			configureContext(context);
 			return context;
diff --git a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ApplicationContextRunner.java b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ApplicationContextRunner.java
index a45fd753c2..feb9416b6c 100644
--- a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ApplicationContextRunner.java
+++ b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ApplicationContextRunner.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -59,22 +59,24 @@ public class ApplicationContextRunner extends
 	}
 
 	private ApplicationContextRunner(Supplier<ConfigurableApplicationContext> contextFactory,
+			boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		super(contextFactory, initializers, environmentProperties, systemProperties, classLoader, parent,
-				beanRegistrations, configurations);
+		super(contextFactory, allowBeanDefinitionOverriding, initializers, environmentProperties, systemProperties,
+				classLoader, parent, beanRegistrations, configurations);
 	}
 
 	@Override
 	protected ApplicationContextRunner newInstance(Supplier<ConfigurableApplicationContext> contextFactory,
+			boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		return new ApplicationContextRunner(contextFactory, initializers, environmentProperties, systemProperties,
-				classLoader, parent, beanRegistrations, configurations);
+		return new ApplicationContextRunner(contextFactory, allowBeanDefinitionOverriding, initializers,
+				environmentProperties, systemProperties, classLoader, parent, beanRegistrations, configurations);
 	}
 
 }
diff --git a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ReactiveWebApplicationContextRunner.java b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ReactiveWebApplicationContextRunner.java
index 7f05a8743f..932e27ff85 100644
--- a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ReactiveWebApplicationContextRunner.java
+++ b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ReactiveWebApplicationContextRunner.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -59,23 +59,24 @@ public final class ReactiveWebApplicationContextRunner extends
 	}
 
 	private ReactiveWebApplicationContextRunner(Supplier<ConfigurableReactiveWebApplicationContext> contextFactory,
+			boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableReactiveWebApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		super(contextFactory, initializers, environmentProperties, systemProperties, classLoader, parent,
-				beanRegistrations, configurations);
+		super(contextFactory, allowBeanDefinitionOverriding, initializers, environmentProperties, systemProperties,
+				classLoader, parent, beanRegistrations, configurations);
 	}
 
 	@Override
 	protected ReactiveWebApplicationContextRunner newInstance(
-			Supplier<ConfigurableReactiveWebApplicationContext> contextFactory,
+			Supplier<ConfigurableReactiveWebApplicationContext> contextFactory, boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableReactiveWebApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		return new ReactiveWebApplicationContextRunner(contextFactory, initializers, environmentProperties,
-				systemProperties, classLoader, parent, beanRegistrations, configurations);
+		return new ReactiveWebApplicationContextRunner(contextFactory, allowBeanDefinitionOverriding, initializers,
+				environmentProperties, systemProperties, classLoader, parent, beanRegistrations, configurations);
 	}
 
 }
diff --git a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java
index 1df1bca5fc..8f9c671cd0 100644
--- a/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java
+++ b/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -63,22 +63,24 @@ public final class WebApplicationContextRunner extends
 	}
 
 	private WebApplicationContextRunner(Supplier<ConfigurableWebApplicationContext> contextFactory,
+			boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableWebApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		super(contextFactory, initializers, environmentProperties, systemProperties, classLoader, parent,
-				beanRegistrations, configurations);
+		super(contextFactory, allowBeanDefinitionOverriding, initializers, environmentProperties, systemProperties,
+				classLoader, parent, beanRegistrations, configurations);
 	}
 
 	@Override
 	protected WebApplicationContextRunner newInstance(Supplier<ConfigurableWebApplicationContext> contextFactory,
+			boolean allowBeanDefinitionOverriding,
 			List<ApplicationContextInitializer<? super ConfigurableWebApplicationContext>> initializers,
 			TestPropertyValues environmentProperties, TestPropertyValues systemProperties, ClassLoader classLoader,
 			ApplicationContext parent, List<BeanRegistration<?>> beanRegistrations,
 			List<Configurations> configurations) {
-		return new WebApplicationContextRunner(contextFactory, initializers, environmentProperties, systemProperties,
-				classLoader, parent, beanRegistrations, configurations);
+		return new WebApplicationContextRunner(contextFactory, allowBeanDefinitionOverriding, initializers,
+				environmentProperties, systemProperties, classLoader, parent, beanRegistrations, configurations);
 	}
 
 	/**
diff --git a/spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunnerTests.java b/spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunnerTests.java
index f87e2c0a07..167adc44a7 100644
--- a/spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunnerTests.java
+++ b/spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunnerTests.java
@@ -23,6 +23,7 @@ import java.util.concurrent.atomic.AtomicBoolean;
 import com.google.gson.Gson;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.beans.factory.BeanDefinitionStoreException;
 import org.springframework.boot.context.annotation.UserConfigurations;
 import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.assertj.ApplicationContextAssertProvider;
@@ -139,24 +140,6 @@ abstract class AbstractApplicationContextRunnerTests<T extends AbstractApplicati
 		get().withBean(String.class, () -> "foo").run((context) -> assertThat(context).hasBean("string"));
 	}
 
-	@Test
-	void runWithUserBeanShouldBeRegisteredInOrder() {
-		get().withBean(String.class, () -> "one").withBean(String.class, () -> "two")
-				.withBean(String.class, () -> "three").run((context) -> {
-					assertThat(context).hasBean("string");
-					assertThat(context.getBean("string")).isEqualTo("three");
-				});
-	}
-
-	@Test
-	void runWithConfigurationsAndUserBeanShouldRegisterUserBeanLast() {
-		get().withUserConfiguration(FooConfig.class).withBean("foo", String.class, () -> "overridden")
-				.run((context) -> {
-					assertThat(context).hasBean("foo");
-					assertThat(context.getBean("foo")).isEqualTo("overridden");
-				});
-	}
-
 	@Test
 	void runWithMultipleConfigurationsShouldRegisterAllConfigurations() {
 		get().withUserConfiguration(FooConfig.class).withConfiguration(UserConfigurations.of(BarConfig.class))
@@ -188,6 +171,34 @@ abstract class AbstractApplicationContextRunnerTests<T extends AbstractApplicati
 				.withMessageContaining("Expected message"));
 	}
 
+	@Test
+	void runDisablesBeanOverridingByDefault() {
+		get().withUserConfiguration(FooConfig.class).withBean("foo", Integer.class, () -> 42).run((context) -> {
+			assertThat(context).hasFailed();
+			assertThat(context.getStartupFailure()).isInstanceOf(BeanDefinitionStoreException.class)
+					.hasMessageContaining("Invalid bean definition with name 'foo'")
+					.hasMessageContaining("@Bean definition illegally overridden by existing bean definition");
+		});
+	}
+
+	@Test
+	void runWithUserBeanShouldBeRegisteredInOrder() {
+		get().withAllowBeanDefinitionOverriding(true).withBean(String.class, () -> "one")
+				.withBean(String.class, () -> "two").withBean(String.class, () -> "three").run((context) -> {
+					assertThat(context).hasBean("string");
+					assertThat(context.getBean("string")).isEqualTo("three");
+				});
+	}
+
+	@Test
+	void runWithConfigurationsAndUserBeanShouldRegisterUserBeanLast() {
+		get().withAllowBeanDefinitionOverriding(true).withUserConfiguration(FooConfig.class)
+				.withBean("foo", String.class, () -> "overridden").run((context) -> {
+					assertThat(context).hasBean("foo");
+					assertThat(context.getBean("foo")).isEqualTo("overridden");
+				});
+	}
+
 	protected abstract T get();
 
 	private static void throwCheckedException(String message) throws IOException {