Our Server Validation server behavior that is included in Data Bridge has a "Check for unique value" validation you could use to show an error message when someone tries to insert a repeated value.
Alternatively you could just add a recordset to the top of the page and filter by the value you don't want to allow repeats and only do the database insert/update if the recordset is empty (no repeats found) and show an error message if it has a result (repeat found).