Parent-Child with a unique child identifier

I have the following situation:
A Buyer table (Primary Key = BuyerCode) and a ShipAddress table (Foreign
Key = BuyerCode, and ShipID).

The tables are set up in a normal parent-child relationship, so that
ShipAddr.MasterSource := dsBuyer;
ShipAddr.MasterFields := BuyerCode;

The key stipulation is that ShipID must be unique for the ENTIRE table,
not just for each BuyerCode level. I have tried using a ShipID index
(unique) for ShipAddr.IndexName, but I get a strange effect when I try
to type a value into the ShippingID edit field: As soon as I start to
type in a value, the Buyer Code gets appended to the text!!!

BTW, I am using client data sets, D5.

How do I check that the user's entry for ShipId  is unique for the
entire table.

Any advice would be appreciated