<p>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 <a href="http://blockchain.info/double-spends">most recent double-spends</a>.</p>

<p>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).</p>

<p>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.</p>

CC BY-SA 3.0
2012-05-29T17:34:36.297Z
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
CC BY-SA 3.0
2012-05-29T20:18:01.290Z
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
CC BY-SA 3.0
2012-05-29T22:01:23.580Z
When he says "an example on blockchain" perhaps he means on blockchain.info - I've seen it claim transactions were potential double spends before.
Chris Moore
CC BY-SA 3.0
2012-05-30T13:49:56.450Z
Ah yes sorry, I did mean blockchain.info. However, ThePiachu has explained it well without it.
user545139
CC BY-SA 3.0
2012-05-29T17:17:59.313Z
2012-05-29T17:17:59.313Z
ThePiachu
<p>What does a double spend look like? Can anyone link to an example on <a href="http://blockchain.info/" rel="nofollow">blockchain.info</a>?</p>

<p>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?</p>

<blockchain><bitcoin-core><bitcoind><attack><doublespend>
What does a double spend look like?
user545139
# What does a double spend look like?

(Asked by: username_0 on 2012-05-29)

What does a double spend look like? Can anyone link to an example on [blockchain.info](http://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?

**Tags:** blockchain, bitcoin-core, bitcoind, attack, doublespend


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## 1 answers

### Answer by username_1 on 2012-05-29. Score: 6 | Accepted answer

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](http://blockchain.info/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.

> **Comments:**
> **username_0**: 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?  
> **username_1**: 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.  
> **username_2**: When he says "an example on blockchain" perhaps he means on blockchain.info - I've seen it claim transactions were potential double spends before.  
> **username_0**: Ah yes sorry, I did mean blockchain.info. However, username_1 has explained it well without it.  

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