fix instrumentation module not loading silently when duplicate helper…

muzzle/src/main/java/io/opentelemetry/javaagent/tooling/HelperInjector.java

@@ -117,6 +117,7 @@ public HelperInjector(
 
     List<HelperClassDefinition> helpers =
         helperClassNames.stream()
+            .distinct()
             .map(
                 className ->
                     HelperClassDefinition.create(

testing-common/integration-tests/src/main/java/helper/DuplicateHelper.java

@@ -0,0 +1,14 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package helper;
+
+public class DuplicateHelper {
+  public static String addSuffix(String string, String suffix) {
+    return string + suffix;
+  }
+
+  private DuplicateHelper() {}
+}

testing-common/integration-tests/src/main/java/helper/DuplicateHelperInstrumentation.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package helper;
+
+import static net.bytebuddy.matcher.ElementMatchers.named;
+
+import io.opentelemetry.javaagent.extension.instrumentation.TypeInstrumentation;
+import io.opentelemetry.javaagent.extension.instrumentation.TypeTransformer;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+public class DuplicateHelperInstrumentation implements TypeInstrumentation {
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return named("helper.DuplicateHelperTestClass");
+  }
+
+  @Override
+  public void transform(TypeTransformer transformer) {
+    transformer.applyAdviceToMethod(named("transform"), this.getClass().getName() + "$TestAdvice");
+  }
+
+  @SuppressWarnings("unused")
+  public static class TestAdvice {
+    @Advice.OnMethodExit(suppress = Throwable.class)
+    static void addSuffix(@Advice.Return(readOnly = false) String string) {
+      string = DuplicateHelper.addSuffix(string, " foo");
+    }
+  }
+}

testing-common/integration-tests/src/main/java/helper/DuplicateHelperInstrumentationModule.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package helper;
+
+import static java.util.Collections.singletonList;
+
+import com.google.auto.service.AutoService;
+import io.opentelemetry.javaagent.extension.instrumentation.InstrumentationModule;
+import io.opentelemetry.javaagent.extension.instrumentation.TypeInstrumentation;
+import java.util.Arrays;
+import java.util.List;
+
+@AutoService(InstrumentationModule.class)
+public class DuplicateHelperInstrumentationModule extends InstrumentationModule {
+  public DuplicateHelperInstrumentationModule() {
+    super("duplicate-helper");
+  }
+
+  @Override
+  public List<String> getAdditionalHelperClassNames() {
+    // muzzle adds the same class as helper, listing it twice to ensure it doesn't fail
+    return Arrays.asList("helper.DuplicateHelper", "helper.DuplicateHelper");
+  }
+
+  @Override
+  public boolean isHelperClass(String className) {
+    return "helper.DuplicateHelper".equals(className);
+  }
+
+  @Override
+  public List<TypeInstrumentation> typeInstrumentations() {
+    return singletonList(new DuplicateHelperInstrumentation());
+  }
+}

testing-common/integration-tests/src/test/java/helper/DuplicateHelperTest.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package helper;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class DuplicateHelperTest {
+
+  @Test
+  void duplicateHelper() {
+    String string = DuplicateHelperTestClass.transform("test");
+    assertThat(string).isEqualTo("test foo");
+  }
+}

testing-common/integration-tests/src/test/java/helper/DuplicateHelperTestClass.java

@@ -0,0 +1,16 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package helper;
+
+class DuplicateHelperTestClass {
+
+  // this method is transformed by instrumentation
+  public static String transform(String string) {
+    return string;
+  }
+
+  private DuplicateHelperTestClass() {}
+}