Unique combo box selection in a continuous form

Currently I'm working on a quote form in an inventory database. The invoice form consists of a main form with the customers information and a subform which is continous and is for selecting parts. Each part has multiple serial numbers which need to be quoted individually. Basically what I want to do is when you select a specific serial number on one record, I want it removed from the combo box on all other records in the continuous form. (either previous or new) How would I go about doing this?

Look at "DemoComboWA2000.mdb"