This type of trigger is not a feature of DataAssist. To check for a duplicate value you will need to manually implement the check. This will consist of having an additional recordset on the page that search in the table for a record where the specified column matches the posted value from the form.
Once you have the recordset in place you can add in a numeric validation based on the total rows variable for that recordset. If this value is greater than 0 then there was a match and the record is a duplicate one. If it is 0 or less then there are no matches and you should proceed with inserting the record.