The combination of Work Order and Serial Numbers must be unique. Define a text field that auto-enters a calculation shuch as:
IL_WO & " " & IL_SerialNumber
and now you can specify unique values on that field.
An even more user friendly approach is to set up an interface where the user can't select the same serial number in the same portal to item list a second time. This can be done with a diminishing value list--a special kind of conditional value list.
This demo file illustrates three approaches to solving your issue, the validation rule, the diminishing value list and a "check boxes" selection format: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7