I want the user to be able to enter the zip codes they want to purchase for an SEO service, but I only want to allow one client per zip.

I've built a form that lets them enter 10 different zips, and it will check against the zips in that same field, but if one person entered that zip in field 1, and the next person entered it in field 2, it wouldn't match them.

Does anyone have any idea how to achieve this?

Thanks