chore: don't add the same Aspect more than once

There is no observable behavior from adding the same Aspect
more than once at the same priority, but we are adding multiple
`AspectApplication`s for it (and then only executing once).

Make the proptest results a bit easier to read by not allowing this in
the first place.

packages/aws-cdk-lib/core/lib/aspect.ts

@@ -82,7 +82,11 @@ export class Aspects {
    * @param options Options to apply on this aspect.
    */
   public add(aspect: IAspect, options?: AspectOptions) {
-    this._appliedAspects.push(new AspectApplication(this._scope, aspect, options?.priority ?? AspectPriority.DEFAULT));
+    const newApplication = new AspectApplication(this._scope, aspect, options?.priority ?? AspectPriority.DEFAULT);
+    if (this._appliedAspects.some(a => a.aspect === newApplication.aspect && a.priority === newApplication.priority)) {
+      return;
+    }
+    this._appliedAspects.push(newApplication);
   }
 
   /**

packages/aws-cdk-lib/core/test/aspect.test.ts

@@ -74,6 +74,20 @@ describe('aspect', () => {
     expect(root.visitCounter).toEqual(1);
   });
 
+  test('Adding the same aspect twice to the same construct only adds 1', () => {
+    // GIVEN
+    const app = new App();
+    const root = new MyConstruct(app, 'MyConstruct');
+
+    // WHEN
+    const asp = new VisitOnce();
+    Aspects.of(root).add(asp);
+    Aspects.of(root).add(asp);
+
+    // THEN
+    expect(Aspects.of(root).all.length).toEqual(1);
+  });
+
   test('if stabilization is disabled, warn if an Aspect is added via another Aspect', () => {
     const app = new App({ context: { '@aws-cdk/core:aspectStabilization': false } });
     const root = new MyConstruct(app, 'MyConstruct');
@@ -91,7 +105,7 @@ describe('aspect', () => {
     expect(root.node.metadata[0].type).toEqual(cxschema.ArtifactMetadataEntryType.WARN);
     expect(root.node.metadata[0].data).toEqual('We detected an Aspect was added via another Aspect, and will not be applied [ack: @aws-cdk/core:ignoredAspect]');
     // warning is not added to child construct
-    expect(child.node.metadata.length).toEqual(0);
+    expect(child.node.metadata).toEqual([]);
   });
 
   test('Do not warn if an Aspect is added directly (not by another aspect)', () => {