fix(pipelines): graphnode dependencies can have duplicates (aws#18450)

`GraphNode.allDeps` allows duplicate dependencies to be returned. This does not have any affect on the performance of the pipelines module, but looks ugly. This was noticed in cdklabs/cdk-pipelines-github#67, where the dependencies are written out to the `deploy.yaml` file. 

I did not change the underlying `GraphNode.dependencies` structure to be a set (although I think it should) because I feel like that is a breaking change. So instead I've preserved the structure of the API and deduplicated the list of GraphNodes.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/pipelines/lib/helpers-internal/graph.ts

@@ -35,7 +35,7 @@ export class GraphNode<A> {
    */
   public get allDeps(): GraphNode<A>[] {
     const fromParent = this.parentGraph?.allDeps ?? [];
-    return [...this.dependencies, ...fromParent];
+    return Array.from(new Set([...this.dependencies, ...fromParent]));
   }
 
   public dependOn(...dependencies: Array<GraphNode<A> | undefined>) {
@@ -382,4 +382,4 @@ function projectDependencies<A>(dependencies: Map<GraphNode<A>, Set<GraphNode<A>
 
 export function isGraph<A>(x: GraphNode<A>): x is Graph<A> {
   return x instanceof Graph;
-}
+}

packages/@aws-cdk/pipelines/test/blueprint/helpers-internal/dependencies.test.ts

@@ -50,3 +50,22 @@ describe('with nested graphs', () => {
     ]);
   });
 });
+
+test('duplicate dependencies are ignored', () => {
+  mkGraph('G', G => {
+    const A = G.graph('A', [], GA => {
+      GA.node('aa');
+    });
+
+    // parent graph depnds on A also
+    const B = G.graph('B', [A], GB => {
+      // duplicate dependency on A
+      GB.graph('BB', [A], GBB => {
+        GBB.node('bbb');
+      });
+      GB.node('bb');
+    });
+
+    expect(nodeNames(B.tryGetChild('BB')!.allDeps)).toStrictEqual(['A']);
+  });
+});