The Email address is already taken. Specify a different email address for the user

The Email address is already taken. Specify a different email address for the user

I am trouble adding a user. Getting error message "The Email address is already taken. Specify a different email address for the user". What can I do? The emaill address I am trying to addd is - azrin.ibrahim@brunei-halal.com - khairul.alimin@brunei-halal.com