Make user_email field unique?

Hello,

I’m trying to integrate my WordPress self hosted blog with a Laconi.ca installation.

There are many differences beetween those two platforms but above all I need to make wordpress user email address an unique index. In fact even if during registration wordpress checks about existing email addresses, any user is able to modify its own address from its profile and in this case there’s no check for duplicates.

Setting it up on Mysql DB is quite easy, hovever wordpress do not give any error/alert when I insert an email address that already exists, even if mysql blocks it.

Is there any clean way to do so?