Your Best Buyer Was Sitting in Eight Ad Sets, and You Paid for All Eight.
Overlapping audiences across ad sets can bill an account for the same buyer more than once, and nothing about the invoice ever says so.
One buyer, three ad sets, three charges
Under a funnel built from three ad sets, top, middle, and bottom, a single real person can sit inside all three groups at once. Each ad set independently serves that person, counts the impression as its own reach, and treats the resulting purchase as a fresh acquisition. The account ends up buying the same human being multiple times through different doors, and no single line item ever says so.
Opening 25 doors emptied half the room
One documented restructure replaced that three-ad-set funnel with 25 concept-based ad sets. Meta consolidated each person toward whichever concept they actually engaged with, and roughly half the account's reported audience evaporated in the process, because half of it had been the same people counted more than once. The operator's own analogy: the old structure was a prison with three cells; the new one opened 25 doors, everyone tried every door for a week, then each person found the room they actually wanted and stayed there.
No invoice ever itemizes this
Nothing about this shows up as a billing anomaly, because from the platform's side nothing went wrong. Overlapping reach across ad sets is just how the auction works when audiences aren't mutually exclusive, and the platform has no reason to itemize the redundancy it happens to be selling. The only way to see it is a record of who actually converts, built and owned independently of how many ad sets happened to reach them along the way, so a duplicated buyer shows up as one customer instead of two or three billable events.