In 6.10 we have a feature do find duplicates.

Do you want to act on duplicates or do you want to forbid registering a second time when the user can't figure out his password? (or forgot that he already registered sometime a long time ago...)

I like to forbidden user to register two times whit same email address. Because if user is forgot password it can reset it, I think that two accounts whit same email address has make no sense.

Ok, we'll check that.