Actually, if there is a user with a registered email, and a guest uses the same email address to post a topic or post, this won't affect forum statistics. That is, a second user won't be created with the same email, and the post won't be attached to the current user.

Is it opssible to forbid the use of an email address for anonymous users if email is already used by a registered member ?

This is possible because a registered user may want to post anonymously without being logged in. In this case, they should have the opportunity to post using the same email address as a guest or anonymously.

I authorize anonymous posting on my forum.

I have registered WP members too.

As an anonymous user, I can use the same email address that a registered user, so 2 users can be seen as the same user.

Pretty confusing.

Is it opssible to forbid the use of an email address for anonymous users if email is already used by a registered member ?

Thanks.