Creating New User Account in Admin Allows Existing Email Address Without Warning

There is no warning when creating accounts that the email address provided already belongs to an existing user.

1. I created a new account with a new and unique username, but used an email belonging to an existing account.
2. I expected the Admin would warn me that the email address is already in use.
3. The Admin went ahead without any warning and created a new account with an existing new email address (one that belongs to another account).