Duplicate or split, and when to use each
Duplicating the same creatives into every ad set is how you learn which audience responds to which creative, because the creative variable is held constant. Splitting creatives across ad sets is how you run several independent tests at once when budget is tight and you cannot afford every creative in every audience. The mistake is doing one while believing you did the other: if creatives differ between ad sets, differences in performance tell you nothing clean about the audiences.