Can I search a user custom field?

I'm doing a registration form with an hidden field for an ID already filled. Every user has one. But in order to not have ID duplicates, i have to search on the users fields and block the form if found registered, something like:

if user field ID contains "123456" the deny, else show form

how can I do that?

Answered by Vitor Pinho!
>>>>>>> Answered <<<<<<<
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