In the list of fields, it looks like if do distinct by name

[vsegura]

This question is in reference to Atlassian Documentation: Using validators with custom fields

It list all custom fields and then distinct by name. It was wrong, because, JIRA able us to have many custom field with the same name.

[Nic Brough -Adaptavist-]

JIRA tries to stop you creating fields with the same name, because it's a spectacularly dreadful thing to do.  But there are a number of ways of doing it by accident.

The documentation is essentially correct, because you should not have many fields with the same name, and validator code doesn't support it.