Preconditions:

Application opened in two separate windows. Tested with browsersync with two opened browser windows.

Steps:

1. Go to Admin/Users/Administrator Users
2. Click Create and choose User
3. Fill required fields and click Create

Actual result:

One instance of applications create user successfully, but the other one shows database error (500).

Expected result:

Error validation message on the user field should be displayed with the following message:
The username already exists. Please use another username