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.

moto/cognitoidp/exceptions.py

@@ -24,6 +24,16 @@ class UserNotFoundError(BadRequest):
         })
 
 
+class UsernameExistsException(BadRequest):
+
+    def __init__(self, message):
+        super(UsernameExistsException, self).__init__()
+        self.description = json.dumps({
+            "message": message,
+            '__type': 'UsernameExistsException',
+        })
+
+
 class GroupExistsException(BadRequest):
 
     def __init__(self, message):

moto/cognitoidp/models.py

@@ -14,7 +14,8 @@ from jose import jws
 
 from moto.compat import OrderedDict
 from moto.core import BaseBackend, BaseModel
-from .exceptions import GroupExistsException, NotAuthorizedError, ResourceNotFoundError, UserNotFoundError
+from .exceptions import GroupExistsException, NotAuthorizedError, ResourceNotFoundError, UserNotFoundError, \
+    UsernameExistsException
 
 UserStatus = {
     "FORCE_CHANGE_PASSWORD": "FORCE_CHANGE_PASSWORD",
@@ -561,6 +562,9 @@ class CognitoIdpBackend(BaseBackend):
         if not user_pool:
             raise ResourceNotFoundError(user_pool_id)
 
+        if username in user_pool.users:
+            raise UsernameExistsException(username)
+
         user = CognitoIdpUser(user_pool_id, username, temporary_password, UserStatus["FORCE_CHANGE_PASSWORD"], attributes)
         user_pool.users[user.username] = user
         return user