Hide Used Items in Drop Down List across multiple sheets

You were very close....

You did not specify how many DD boxes could appear on each sheet. So I gave you 9...

I changed the formulae in the

B2: =SUMPRODUCT(COUNTIF(INDIRECT("'"&Sheetlist&"'! C2:C10"),A2))
where "Sheetlist" is a Named range, referring to the orange shaded list of sheets where the DD boxes are to be located. Use CTRL-F3 to view/edit.

C2: =IFERROR(INDEX(A:A,AGGREGATE(15,6,ROW($A$2:$A$10)/($B$2:$B$10=0),ROWS(C$2:C2))),"")

Then just set up the DV cells on each sheet. Job done.