When I applied I had my first offer in December, second in February and last one in March. I personally didn't withdraw even though the last one was the same offer as my insurance.
They aren't going to give out more offers because X person has withdrawn. There is the every so slight positive that if you fail your offer you have one more university to grovel to. There is literally no positive to withdrawing. In the end I had an AAA firm and an AAB insurance. Had my firm and insurance offer been the same I'd still have held out as if I'd got AAB I'd have had two universities to have the slim chance of groveling to. A slim chance is better than no chance.