Avoid duplicates in MergedContextConfiguration

Remove duplicate config attribute classes by using a `Set` rather
than a `List` in the `SpringBootTestContextBootstrapper`.

See gh-24532

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTestContextBootstrapper.java

@@ -134,7 +134,7 @@ protected ContextLoader resolveContextLoader(Class<?> testClass,
 	}
 
 	private void addConfigAttributesClasses(ContextConfigurationAttributes configAttributes, Class<?>[] classes) {
-		List<Class<?>> combined = new ArrayList<>(Arrays.asList(classes));
+		Set<Class<?>> combined = new LinkedHashSet<>(Arrays.asList(classes));
 		if (configAttributes.getClasses() != null) {
 			combined.addAll(Arrays.asList(configAttributes.getClasses()));
 		}

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/bootstrap/SpringBootTestContextBootstrapperTests.java

@@ -23,6 +23,7 @@
 import org.springframework.boot.test.context.SpringBootTestContextBootstrapper;
 import org.springframework.test.context.BootstrapContext;
 import org.springframework.test.context.CacheAwareContextLoaderDelegate;
+import org.springframework.test.context.MergedContextConfiguration;
 import org.springframework.test.context.TestContext;
 import org.springframework.test.context.web.WebAppConfiguration;
 import org.springframework.test.util.ReflectionTestUtils;
@@ -56,39 +57,47 @@ void springBootTestWithAMockWebEnvironmentCanBeUsedWithWebAppConfiguration() {
 	@Test
 	void mergedContextConfigurationWhenArgsDifferentShouldNotBeConsideredEqual() {
 		TestContext context = buildTestContext(SpringBootTestArgsConfiguration.class);
-		Object contextConfiguration = ReflectionTestUtils.getField(context, "mergedContextConfiguration");
+		MergedContextConfiguration contextConfiguration = getMergedContextConfiguration(context);
 		TestContext otherContext2 = buildTestContext(SpringBootTestOtherArgsConfiguration.class);
-		Object otherContextConfiguration = ReflectionTestUtils.getField(otherContext2, "mergedContextConfiguration");
+		MergedContextConfiguration otherContextConfiguration = getMergedContextConfiguration(otherContext2);
 		assertThat(contextConfiguration).isNotEqualTo(otherContextConfiguration);
 	}
 
 	@Test
 	void mergedContextConfigurationWhenArgsSameShouldBeConsideredEqual() {
 		TestContext context = buildTestContext(SpringBootTestArgsConfiguration.class);
-		Object contextConfiguration = ReflectionTestUtils.getField(context, "mergedContextConfiguration");
+		MergedContextConfiguration contextConfiguration = getMergedContextConfiguration(context);
 		TestContext otherContext2 = buildTestContext(SpringBootTestSameArgsConfiguration.class);
-		Object otherContextConfiguration = ReflectionTestUtils.getField(otherContext2, "mergedContextConfiguration");
+		MergedContextConfiguration otherContextConfiguration = getMergedContextConfiguration(otherContext2);
 		assertThat(contextConfiguration).isEqualTo(otherContextConfiguration);
 	}
 
 	@Test
 	void mergedContextConfigurationWhenWebEnvironmentsDifferentShouldNotBeConsideredEqual() {
 		TestContext context = buildTestContext(SpringBootTestMockWebEnvironmentConfiguration.class);
-		Object contextConfiguration = ReflectionTestUtils.getField(context, "mergedContextConfiguration");
+		MergedContextConfiguration contextConfiguration = getMergedContextConfiguration(context);
 		TestContext otherContext = buildTestContext(SpringBootTestDefinedPortWebEnvironmentConfiguration.class);
-		Object otherContextConfiguration = ReflectionTestUtils.getField(otherContext, "mergedContextConfiguration");
+		MergedContextConfiguration otherContextConfiguration = getMergedContextConfiguration(otherContext);
 		assertThat(contextConfiguration).isNotEqualTo(otherContextConfiguration);
 	}
 
 	@Test
-	void mergedContextConfigurationWhenWebEnvironmentsSameShouldtBeConsideredEqual() {
+	void mergedContextConfigurationWhenWebEnvironmentsSameShouldBeConsideredEqual() {
 		TestContext context = buildTestContext(SpringBootTestMockWebEnvironmentConfiguration.class);
-		Object contextConfiguration = ReflectionTestUtils.getField(context, "mergedContextConfiguration");
+		MergedContextConfiguration contextConfiguration = getMergedContextConfiguration(context);
 		TestContext otherContext = buildTestContext(SpringBootTestAnotherMockWebEnvironmentConfiguration.class);
-		Object otherContextConfiguration = ReflectionTestUtils.getField(otherContext, "mergedContextConfiguration");
+		MergedContextConfiguration otherContextConfiguration = getMergedContextConfiguration(otherContext);
 		assertThat(contextConfiguration).isEqualTo(otherContextConfiguration);
 	}
 
+	@Test
+	void mergedContextConfigurationClassesShouldNotContainDuplicates() {
+		TestContext context = buildTestContext(SpringBootTestClassesConfiguration.class);
+		MergedContextConfiguration contextConfiguration = getMergedContextConfiguration(context);
+		Class<?>[] classes = contextConfiguration.getClasses();
+		assertThat(classes).containsExactly(SpringBootTestContextBootstrapperExampleConfig.class);
+	}
+
 	@SuppressWarnings("rawtypes")
 	private TestContext buildTestContext(Class<?> testClass) {
 		SpringBootTestContextBootstrapper bootstrapper = new SpringBootTestContextBootstrapper();
@@ -100,6 +109,10 @@ private TestContext buildTestContext(Class<?> testClass) {
 		return bootstrapper.buildTestContext();
 	}
 
+	private MergedContextConfiguration getMergedContextConfiguration(TestContext context) {
+		return (MergedContextConfiguration) ReflectionTestUtils.getField(context, "mergedContextConfiguration");
+	}
+
 	@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
 	@WebAppConfiguration
 	static class SpringBootTestNonMockWebEnvironmentAndWebAppConfiguration {
@@ -142,4 +155,9 @@ static class SpringBootTestOtherArgsConfiguration {
 
 	}
 
+	@SpringBootTest(classes = SpringBootTestContextBootstrapperExampleConfig.class)
+	static class SpringBootTestClassesConfiguration {
+
+	}
+
 }