We track and collect double spend attempts on the Bitcoin Cash network.
A zero-conf, also known an unconfirmed transaction, is a transcaction that has not been mined into a block by a Bitcoin miner.
When a transaction is mined into a block, it's a confirmed transaction.
Double spend is when someone attempts to spend the same Bitcoin to two different addresses. These exist as two (or more) transactions that have not yet been mined (also known as zero-confs)
While both transactions are for the moment valid, when one of them gets mined into a block the other one gets invalidated. Both cannot be mined. In most cases, the first transaction seen on the network is mined (wins), as this website demonstrates.
Double spend can happen for various reasons. A wallet may not be fully synced. Software bugs. Someone having fun. One of the reasons can is fraud attempts.
Double spending attempts are relatively rare compared to the total amount of transactions on the network.
If it fits your risk profile, accepting zero-conf payments will give you the benefit of instant transactions.
For many applications, accepting an zero-conf transaction may be acceptable risk. For example for smaller amounts, or when knowing the sender in person.
For larger amount, the stakes are higher and the benefits of attempting to defraud you are higher. Under such circumstances you may want to wait for block confirmations.
After receiving a transaction, if you monitor the network for double spends for only a few seconds before accepting it as valid payment, you'll drastically reduce the acceptance risk, as this is the time needed for most of the network to see your transaction firstBitcoin Unlimited nodes, which account for half of the nodes on the network support forwarding double spending transactions, allowing for network wide detection of double spends. This site detects double spends thanks to this technology.
Rather than relaying the double spends themselves, a proof is generated and relayed to warn the network that there exists a double spend of a transaction.
Specification. Discontinued on Bitcoin Cash, however, exists deployed on an altcoin called Chain2.