Make Action#addOutputArtifact idempotent.

packages/@aws-cdk/aws-codepipeline-api/lib/action.ts

@@ -252,6 +252,13 @@ export abstract class Action {
   }
 
   protected addOutputArtifact(name: string): Artifact {
+    // adding the same name multiple times doesn't do anything -
+    // addOutputArtifact is idempotent
+    const ret = this._outputArtifacts.find(output => output.artifactName === name);
+    if (ret) {
+      return ret;
+    }
+
     const artifact = new Artifact(name);
     this._actionOutputArtifacts.push(artifact);
     return artifact;

packages/@aws-cdk/aws-codepipeline/test/test.action.ts

@@ -185,6 +185,28 @@ export = {
       test.done();
     },
   },
+
+  'output Artifact names': {
+    'accept the same name multiple times safely'(test: Test) {
+      const artifact = new actions.Artifact('SomeArtifact');
+
+      const stack = new cdk.Stack();
+      const project = new codebuild.PipelineProject(stack, 'Project');
+      const action = project.toCodePipelineBuildAction({
+        actionName: 'CodeBuild',
+        inputArtifact: artifact,
+        outputArtifactName: 'Artifact1',
+        additionalOutputArtifactNames: [
+          'Artifact1',
+          'Artifact1',
+        ],
+      });
+
+      test.equal(action._outputArtifacts.length, 1);
+
+      test.done();
+    },
+  },
 };
 
 function boundsValidationResult(numberOfArtifacts: number, min: number, max: number): string[] {