Existing user now raises a UsernameExistsException

If a user is attempted to be added to a pool that already contains a user with that username, the UsernameExistsException is thrown, to match AWS behaviour.

tests/test_cognitoidp/test_cognitoidp.py

@@ -886,6 +886,36 @@ def test_admin_create_user():
     result["User"]["Enabled"].should.equal(True)
 
 
+@mock_cognitoidp
+def test_admin_create_existing_user():
+    conn = boto3.client("cognito-idp", "us-west-2")
+
+    username = str(uuid.uuid4())
+    value = str(uuid.uuid4())
+    user_pool_id = conn.create_user_pool(PoolName=str(uuid.uuid4()))["UserPool"]["Id"]
+    conn.admin_create_user(
+        UserPoolId=user_pool_id,
+        Username=username,
+        UserAttributes=[
+            {"Name": "thing", "Value": value}
+        ],
+    )
+
+    caught = False
+    try:
+        conn.admin_create_user(
+            UserPoolId=user_pool_id,
+            Username=username,
+            UserAttributes=[
+                {"Name": "thing", "Value": value}
+            ],
+        )
+    except conn.exceptions.UsernameExistsException:
+        caught = True
+
+    caught.should.be.true
+
+
 @mock_cognitoidp
 def test_admin_get_user():
     conn = boto3.client("cognito-idp", "us-west-2")