fix(cli): listing same stack multiple times fails (#10554)

Fixes listing same stack multiple times fails for cdk commands

closes #9667

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

packages/aws-cdk/lib/api/cxapp/cloud-assembly.ts

@@ -69,6 +69,7 @@ export class CloudAssembly {
 
   public async selectStacks(selectors: string[], options: SelectStacksOptions): Promise<StackCollection> {
     selectors = selectors.filter(s => s != null); // filter null/undefined
+    selectors = [...new Set(selectors)]; // make them unique
 
     const stacks = this.assembly.stacks;
     if (stacks.length === 0) {

packages/aws-cdk/test/api/cloud-assembly.test.ts

@@ -60,6 +60,19 @@ test('select behavior: single', async () => {
     .rejects.toThrow('Since this app includes more than a single stack, specify which stacks to use (wildcards are supported)');
 });
 
+test('select behavior: repeat', async () => {
+  // GIVEN
+  const cxasm = await testCloudAssembly();
+
+  // WHEN
+  const x = await cxasm.selectStacks(['withouterrors', 'withouterrors'], {
+    defaultBehavior: DefaultSelection.AllStacks,
+  });
+
+  // THEN
+  expect(x.stackCount).toBe(1);
+});
+
 async function testCloudAssembly({ env }: { env?: string, versionReporting?: boolean } = {}) {
   const cloudExec = new MockCloudExecutable({
     stacks: [{