to prevent duplicates, you would need to create a recordset. Filter the column you do not what duplicates for on the form element duplicates should not be entered into. Name the rcordset "CheckRepeat".
Then add server side validation. select numeric validation, for the server variable, enter "$totalRows_CheckRepeat" and click the plus button to add it. Set the Minimum to -1 and the max to 0.
This will cause validation to fail if there are any records in the CheckRepeat recordset.