Are you using LIST-ITEM-PAIRS?
Is it duplicate codes that you want to avoid or duplicate displayed values?
Whichever method, you should be able to find a solution using either the LOOKUP attribute of the combo box, or the LOOKUP function on the combo's LIST-ITEMS or LIST-ITEM-PAIRS attribute.
You would have to build this into your combo box load.
e.g.
FOR EACH Table:
IF cbox:LOOKUP(Table.Field) EQ 0
THEN cbox:ADD-LAST(Table.Field).
END.