fix(iam): cannot add multiple conditions using same operator

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packages/@aws-cdk/aws-iam/lib/policy-statement.ts

@@ -167,7 +167,8 @@ export class PolicyStatement {
    * Add a condition to the Policy
    */
   public addCondition(key: string, value: any) {
-    this.condition[key] = value;
+    const existingValue = this.condition[key];
+    this.condition[key] = existingValue ? { ...existingValue, ...value } : value;
   }
 
   /**

packages/@aws-cdk/aws-iam/test/policy-document.test.ts

@@ -572,4 +572,19 @@ describe('IAM polocy document', () => {
 
     expect(stack.resolve(doc1)).toEqual(stack.resolve(doc2));
   });
+
+  test('adding another condition with the same operator does not delete the original', () => {
+    const stack = new Stack();
+
+    const p = new PolicyStatement();
+
+    p.addCondition('StringEquals', { 'kms:ViaService': 'service' });
+
+    p.addAccountCondition('12221121221');
+
+    expect(stack.resolve(p.toStatementJson())).toEqual({
+      Effect: 'Allow',
+      Condition: { StringEquals: { 'kms:ViaService': 'service', 'sts:ExternalId': '12221121221' } }
+    });
+  });
 });