New user registering with an existing e-mail

Suppose that a new user goes to register on a site and enters an existing e-mail address. What message should be presented to that user from a UX point of view? Bear in mind that in the above case(as I see it)...two type of users might do the following:

  1. An already registered user who just wants to see what will happen if he attempts to register(that is one thing I possibly can imagine).
  2. A malicious user who knows the e-mail of another person and tries to find out if he/she is registered on the site.

How can I deal with the above cases? Of course I cannot know in advance which type of user goes to fill the form. So I am trying to find a message that will cover both of the above cases.

What do you suggest?