Profile: pedroS-6

Problem:
How to validate CRED form for duplicate postmeta or custom field value

Solution:
You can use CRED hook cred_form_validate in order to validate your form when you submit the form and check if the custom field value for field "user" is already exists or not.

You can find proposed solution, in this case, with the following reply:
=> https://toolset.com/forums/topic/quiero-que-un-campo-sea-unico/#post-740544

Relevant Documentation:
https://toolset.com/documentation/programmer-reference/cred-api/#cred_form_validate