Reject add row if username already exists

Hello.
I am using Tablesome as an event register, a simple NAME|GROUP|ATTENDING table alongside a Forminator form submittion. This works great, however my group of users often submit the form multiple times, creating double or more entries on the table. I’m having to tidy up the table daily by deleting duplicate entries.

Is there a way Tablesome can recognize that a username already exists and either reject, or redirect to prevent multiple entries?

Thanks.