Accepting 0-confirmation transactions and detecting double-spends with open source solution I would like to accept 0-confirmation bitcoin for very small amounts. I am happy to accept zero-confirmation spends and am aware of the risks. However, the tool needs the ability to "flag" the accepted transaction/s if a double-spend is detected. I can then decide what action to take.

I do not want to use a centralized third-party tool. I would like to use something locally on the server. It would simply display pre-generated public addresses. No need for private keys and/or seeds to be on the server.

Does such a tool exist?

Thank you.
You should not interact with bitcoind, because it (current versions at least) filteres double-spend transactions

You should have your own network-sniffer solutution - the program which connects to the network nodes (as many as possible) and listens for all transactions, checking do they have conflicts with particular ones.

(How much would you pay for such program? Just interesting :) )

This is also not 100% safe solution - a miner can place double-spending transaction in block and mine it himself. Look here: https://bitcointalk.org/index.php?topic=327767.0