Double spend is when a set of coins is spent in more than one transaction. This can happen for various reasons, but one of the reasons is fraud attempts
If you are accepting 0-conf payments, it's in your interest to detect double spend attempts. If you want merchants to accept your 0-conf payments, it's in your interest to help detect double spend attempts.
Bitcoin XT and Bitcoin Unlimited detect double spends relayed to it. Bitcoin XT allows you to react to double spends on your wallet. Both nodes also forward double spends to allow other nodes to detect them.
Help gathering double spends here and detecting them network wide by running Bitcoin XT or Bitcoin Unlimited and connecting to at least one other node with relay support, such as respends.thinlink.com:8333
Below are double spends detected with Bitcoin XT. Tags are applied to some as analysis for cause or motivation of the double spend.
Last detected: 2018-11-14 06:36:42.592134