How to create ethereum withdrawal system on a site?

I want to create an ethereum withdrawal system on my site. I know how to send transactions with web3and do other things but I have one problem. If you are trying to send transaction while you already have a processing transaction you will get {'code': -32000, 'message': 'known transaction: (txn hash)'} error. That means that I can't just process transactions without stopping. So how can I do it? Should I make a queue or something like that? I heard I can do it with promises but I don't know how.