What does a double spend look like? Can anyone link to an example on blockchain.info?

Will invalid half of the double spend simply remain at 0-confirmations, or might it ever reach 1 or 2? Will the standard bitcoind client provide any type of flag for known invalid transactions?

1 Answer 1


You won't be able to see a double-spend in the block chain, because it is invalid. You will see one of the transactions included in the chain though. Blockchain.info has a page showing the most recent double-spends.

Double spend is just two (or more) transactions claiming the same input, so they will look like an ordinary pair of transaction. One of them will get included in a block, the other will eventually be forgotten by the network. A conflicting transaction to the one included in a valid block can never become a part of another block (at least until the first block will become invalidated by a 51% attack).

The standard client will not flag any invalid transactions, but it will reject any transaction that conflicts with a known transaction. It will, however, accept a transaction that is a part of a block, even if it conflicts with a 0-confirmation transaction that you know of.

  • So lets say my bitcoin client receives a 0-confirmation transaction. It then accepts ten blocks which have a conflicting transaction. Does the original 0-confirmation transaction remain listed in my client's transactions, or does it just disappear? What if the first transaction had 1 confirmation instead of 0?
    – user545139
    Commented May 29, 2012 at 17:34
  • The transaction will remain in your client, but other clients will start forgetting about it. It will have 0-confirmations. If the first transaction has 1 confirmation, it means it was a part of a block. Should a conflicting block be mined, it will be reverted to 0-confirmations.
    – ThePiachu
    Commented May 29, 2012 at 20:18
  • When he says "an example on blockchain" perhaps he means on blockchain.info - I've seen it claim transactions were potential double spends before. Commented May 29, 2012 at 22:01
  • Ah yes sorry, I did mean blockchain.info. However, ThePiachu has explained it well without it.
    – user545139
    Commented May 30, 2012 at 13:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.