Module also sends mails to mail addresses that are already in by other users

Hi jpetso

great module! One issue I just noticed is that I can send invitations to an e-mail address that is already used by another user on the same site. When the invited user logs in with the code he has the same e-mail address as the previously existing user.

When the previously existing user tries to change his account settings and submits the form he gets an error message that his e-mail address is already used by another users.

I fixed this by checking if the entered value for the e-mail already exits in the temporary_invitation_manage_validate() function. See attached patch file.