I was a bit surprised to find that you didn't have an option for this already since you check if an email address is already registered for the game.
Anyways, to have this option would solve a lot of problems especially with bad users who can now just create a new account in a few seconds using any email address with the correct syntax.
And it's good to have a users real email if you wan't to contact them for offers/issues/information.
I don't think it would be too hard for you to create this since you already have a system for password resets. Hopefully you can add this feature pretty soon.