I have found myself accidentally buying 18 copies of a piece of software. I used 1Password to fill a form out for me and when it auto submitted it changed the quantity without me realising. I had contacted the developer who has apparently had a series of people suffer the same, so I thought I'd note it here.
The form that troubled me is the one after you decide to purchase here: https://store.ergonis.com/products/popcharx
I ended up with 18 of them! Quite a surprise. I hope you can find the issue, not least for the developer's sake!