a transaction can’t be “cancelled”after it is broadcasted.. if there are double spending txs in the mempool, usually the
one with higher fees will be chosen by the miners
Discussion
Ok I’m not good with this so I can explain how it happened. But what I saw on Blue wallet is: I sent the sats to my address but, before any confirmation, there is a Cancel Transaction button that other user with the same seed phrase can click to cancel my transaction and send the sats to his wallet.
i don’t know Blue walllet, but my guess is that this Cancel button does exactly that: generates a replacement transaction with higher fee..
in other words, i don’t think there’s something you can do to avoid this situation.. maybe you could track your and other double spending transactions in the mempool and generate a new one every time you see another with higher fee, until it is included in a block