Prevent Duplicate entry in database by form input

Hi,
I have a form for entrying guests .I want a person can come repeatedly in flat..so security repeated time fill the guest’s details for that same guests is created many times…how i prevent that?..for create guests i can’t see any option for add a custom filter.but whn i use text input in that case create guests have custom filter…



u can make it a true or false stuff

or u do it in such a way that the user can only fill it once