Ignore duplicate schema resource registrations

Closes gh-230

spring-graphql/src/main/java/org/springframework/graphql/execution/DefaultGraphQlSourceBuilder.java

@@ -21,8 +21,10 @@ import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.function.BiFunction;
 import java.util.function.Consumer;
 
@@ -57,7 +59,7 @@ import org.springframework.util.Assert;
  */
 class DefaultGraphQlSourceBuilder implements GraphQlSource.Builder {
 
-	private final List<Resource> schemaResources = new ArrayList<>();
+	private final Set<Resource> schemaResources = new LinkedHashSet<>();
 
 	private final List<RuntimeWiringConfigurer> runtimeWiringConfigurers = new ArrayList<>();
 

spring-graphql/src/test/java/org/springframework/graphql/execution/DefaultGraphQlSourceBuilderTests.java

@@ -26,6 +26,7 @@ import graphql.schema.idl.RuntimeWiring;
 import graphql.schema.idl.WiringFactory;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.graphql.BookSource;
 import org.springframework.graphql.GraphQlSetup;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -38,6 +39,12 @@ import static org.mockito.Mockito.mock;
  */
 public class DefaultGraphQlSourceBuilderTests {
 
+	@Test // gh-230
+	void duplicateResourcesAreIgnored() {
+		// This should not fail with schema errors
+		GraphQlSetup.schemaResource(BookSource.schema, BookSource.schema).toGraphQlSource();
+	}
+
 	@Test
 	void wiringFactoryList() {