http://xi2nhkuzm2hzgyedprs3n65wmudrq4rekbzire67oomiwk5x5h6w7nqd.onion/tr/get-started/faq
In Monero, every @transaction output is uniquely associated with a key image that can only be generated by the holder of that output. Key images that are used more than once are rejected by the miners as double-spends and cannot be added to a valid @block.