Calculating receivables allowance

If bad debts are stated in the trial balance, do we still have to deduct them from the total of the receivables amount when calculating the receivables allowance? My lecturer said yes, but aren’t the bad debts already accounted for in the trial balance (as such the trial balance has to show net amounts of the receivables unless the question mentions that they in fact haven’t been accounted for). I’m asking this hypothetically as I have no question in particular.