Forms :: Prevent Duplicate Entries Of Field On Input Form

I have recently started working for a new company and have inherited a nightmare of a system!

I have basic knowledge of Access and have been asked to stop duplicate entries under the same PO Number.

I have tried using DLookups by looking at other posts on the forum with no success.

The input form is called Purchase Order Entry, with the field called PO_No. The table is called Orders, with the field called Purchase Order Number.

I have tried to make the Purchase Order Number in the Orders table, the primary key but an error that it cannot contain a null value appears.

The Purchase orders may not be entered into the database in numerical order also.