apologies for my pseudo code sql below, but is something like I've described below possible?
My sql isn't quite there yet hence why some syntax is there and some isn't, that's the bit I need your help with....
CHECK ( inserted-display_seq_number (the one being used in the insert) not in (select display_seq_number
where service_group_id = inserted-service_group_id(the one being used in the insert)
and service_package_id = inserted-service_package_id(the one being used in the insert))