Treatment of Bad Debt expense

Hi, I want to ask that when we collect previously written-off receivable, we make entry of Dr Cash Cr Allowance for Doubtful Debts. And, earlier we had already passed an entry of Dr Bad Debt Expense Cr Allowance for Doubtful Debts In the above case, we put the bad debt in allowance account earlier which we later written off as well. However, after some time we received the receivable. In this case, why dont we reverse the expense which we charged off and Dr the allowance for doubtful debts because we have already collected the receivable for which we recorded an allowance. Appreciate any response on this.