The seller is under no obligation to cancel, or reply to messages asking them to do so,especially when ,as mentioned,it is not possible to accidentally commit to buy

They are are also very well within their rights to open a non payment dispute

The seller has already been charged fees for the sale plus potentially lost a real customer and has to wait to be able to resist the item

If you have no use for two items, you could always sell the second one yourself once it arrives