GH-797 - Guard against multiple non-unique module base packages.

spring-modulith-core/src/main/java/org/springframework/modulith/core/AnnotationModulithMetadata.java

@@ -134,8 +134,14 @@ public Optional<String> getSystemName() {
 	@Override
 	public List<String> getBasePackages() {
 
-		var result = new ArrayList<>(List.of(basePackage));
-		result.addAll(List.of(annotation.additionalPackages()));
+		var result = new ArrayList<String>();
+		result.add(basePackage);
+
+		for (var candidate : annotation.additionalPackages()) {
+			if (!result.contains(candidate)) {
+				result.add(candidate);
+			}
+		}
 
 		return result;
 	}

spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java

@@ -141,6 +141,7 @@ protected ApplicationModules(ModulithMetadata metadata, Collection<String> packa
 				pkgs -> importer.importPackages(pkgs).that(not(excluded)), strategy, useFullyQualifiedModuleNames);
 
 		var directSources = packages.stream() //
+				.distinct()
 				.map(it -> JavaPackage.of(classes, it))
 				.flatMap(it -> ApplicationModuleSource.from(it, strategy, useFullyQualifiedModuleNames));
 
@@ -158,6 +159,7 @@ protected ApplicationModules(ModulithMetadata metadata, Collection<String> packa
 				.collect(toMap(ApplicationModule::getName, Function.identity()));
 
 		this.rootPackages = Stream.concat(packages.stream(), contributions.getRootPackages()) //
+				.distinct()
 				.map(it -> JavaPackage.of(classes, it).toSingle()) //
 				.toList();
 

spring-modulith-core/src/test/java/org/springframework/modulith/core/ModulithMetadataUnitTest.java

@@ -66,6 +66,14 @@ public void rejectsTypeNotAnnotatedWithEitherModulithAnnotationOrSpringBootAppli
 				.withMessageContaining(SpringBootApplication.class.getSimpleName());
 	}
 
+	@Test // GH-797
+	void returnsUniqueBasePackages() {
+
+		var metadata = ModulithMetadata.of(AdditionalPackagesShadowing.class);
+
+		assertThat(metadata.getBasePackages()).containsExactly(getClass().getPackageName());
+	}
+
 	@Modulith(additionalPackages = "com.acme.foo", //
 			sharedModules = "shared.module", //
 			systemName = "systemName", //
@@ -82,4 +90,7 @@ static class ModuliticAnnotated {}
 	static class SpringBootApplicationAnnotated {}
 
 	static class Unannotated {}
+
+	@Modulithic(additionalPackages = "org.springframework.modulith.core")
+	static class AdditionalPackagesShadowing {}
 }