Reject identical Bean Overrides

Prior to this commit, the Bean Override feature in the Spring
TestContext Framework (for annotations such as @⁠MockitoBean and
@⁠TestBean) silently allowed one bean override to override another
"identical" bean override; however, Spring Boot's @⁠MockBean and
@⁠SpyBean support preemptively rejects identical overrides and throws
an IllegalStateException to signal the configuration error to the user.

To align with the behavior of @⁠MockBean and @⁠SpyBean in Spring Boot,
and to help developers avoid scenarios that are potentially confusing
or difficult to debug, this commit rejects identical bean overrides in
the Spring TestContext Framework.

Note, however, that it is still possible for a bean override to
override a logically equivalent bean override. For example, a
@⁠TestBean can override a @⁠MockitoBean, and vice versa.

Closes gh-34054

spring-test/src/main/java/org/springframework/test/context/bean/override/BeanOverrideContextCustomizerFactory.java

@@ -24,6 +24,7 @@ import org.springframework.lang.Nullable;
 import org.springframework.test.context.ContextConfigurationAttributes;
 import org.springframework.test.context.ContextCustomizerFactory;
 import org.springframework.test.context.TestContextAnnotationUtils;
+import org.springframework.util.Assert;
 
 /**
  * {@link ContextCustomizerFactory} implementation that provides support for
@@ -51,10 +52,13 @@ class BeanOverrideContextCustomizerFactory implements ContextCustomizerFactory {
 	}
 
 	private void findBeanOverrideHandler(Class<?> testClass, Set<BeanOverrideHandler> handlers) {
-		handlers.addAll(BeanOverrideHandler.forTestClass(testClass));
 		if (TestContextAnnotationUtils.searchEnclosingClass(testClass)) {
 			findBeanOverrideHandler(testClass.getEnclosingClass(), handlers);
 		}
+		BeanOverrideHandler.forTestClass(testClass).forEach(handler ->
+				Assert.state(handlers.add(handler), () ->
+						"Duplicate BeanOverrideHandler discovered in test class %s: %s"
+							.formatted(testClass.getName(), handler)));
 	}
 
 }