Tom, another question if it's possible to do it. Is there any way to make a field unique in order to avoid dupicated records? i.e., make e-mail field unique?

You may want to take a look at CheckUsernameConflict function in web/staff/app/cls_ps_staff.inc.php. The function checkes if the user enters an username that alaready exists in the database. The function is not very general, so you still need to understand how it works and modify it appropretely for your needs.