A user registered with my WordPress blog as "firstname.lastname@example.org" (for example).
I'm fine with that. However, if the user comes back with "email@example.com", I want to stop him for registering again, and maybe politely remind him that he's already registered with the "same" gmail address.
Is there a plugin that does this?
Same question for things like "firstname.lastname@example.org" and "email@example.com" (using extra dots in the email address, which gmail recognizes as the same address).
I realize people can easily get free no-registration-required email addresses, so this won't stop someone from registering multiple times, but it might help.