Because it is just SO annoying, when the motifs are already SO FREAKING RARE to even get ONE, for that one to have an X/14 chance of being a duplicate of one you already know!
Okay, it is less annoying for the Ancestral Orc / Nord / Elf motifs, in the main zones, for which (1) maps are tons more common and (2) there are only 2 possible antiquity leads per zone, 4 in Khenarthi's Roost because Aldmeri Dominion has only a single newbie zone instead of two.
But for the Ancestral Reach (W Skyrim and Reach), Ancestral Akaviri (Blackwood) and Ancient Daedric (Deadlands), it's positively absurd given how rare actual maps for the zones already are: how rare is the chance of actually getting an antiquity lead from a map: for that lead then to be a duplicate of one you already know, when there are some you still need and don't know.
I mean, PLEASE. Can you at least make it so that IF you get a motif lead, it's guaranteed to be one you don't know, unless you know all the antiquity motif leads from that zone?