Agreed, I think it is a pain when, even though you tell members not to sign up more than once, they do it anyway. This would be a nice hack to have but I have a small addition to add to it (you kinda read my mind, I was coming here to post this exact same thing

).
The hack should run a query that would check and list all duplicate IP address, as well as place a checkbox beside them so you can delete them on the spot, so you do not have to go elsewhere in the admin CP to delete them.
SaintDog