A column validation formula to ensure a "Created By" user doesn't submit an item to a list twice?

CristinaMorales Unfortunately, you cannot use the Created By (or any person type column) in SharePoint calculated column formula or list validation formula.

Check if you can implement something by looking at the answer given here: How can I limit user to only enter one item in a list? 

If not, you will have to either customize the list form using Power Apps to check if 1 item by user is already available in list and then do not allow saving new item OR run power automate flow on item creation --> check if item already available --> delete newly created item --> inform user via email.

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