The tool `replace-by-fee-tools` source-code at: https://github.com/petertodd/replace-by-fee-tools Contain several methods for double-spends some links about above repo & more info: https://www.reddit.com/r/Bitcoin/comments/239bj1/doublespending_unconfirmed_transactions_is_a_lot/ https://www.coingecko.com/buzz/peter-todd-explains-how-he-double-spent-coinbase this issue about implementing the techniques in above repo in this app.