Still not very clear for me. But possibly, if you don’t have access to values to filter on them, could you bring them back to the appropriate sheet via a ‘single value’ or a ‘relation’ (possibly via the User sheet), then filter on “relation is not empty” or something like that?
nb: if you want to dynamically block some duplicates in the form, I am not sure that you should use the Glide native Form component; there are threads about this.