Hi,

First, if you want that value to be unique, create an Index in your database for that attribute set as Unique:

After that, if you want to manage that use case in your logic to be more "user friendly", in your action that saves the record, first, get a record with the same number. If you get an empty aggregate, it's ok to save. If it's not empty, trigger a message and don't save. You can log that info too if you want.

Hope this can help.

Best regards,

Ricardo M Pereira