<p>All valid transactions are relayed through the network. A client does not know which transactions will be a part of a block, so it stores and relays both double-spent transactions. If you regularly poll your bitcoind, you can see every transaction before it is put in a block. Then it is just a matter of checking txin data for duplicate inputs.</p>

CC BY-SA 3.0
2012-03-02T08:46:37.277Z
2012-03-02T08:46:37.277Z
ThePiachu
<p>The bitcoin network does NOT relay unconfirmed transactions that double spend other unconfirmed transactions, so neither blockchain.info not anyone else sees most unconfirmed double-spends.  Nodes do not store two conflicting spends - they store only the first one they see.</p>

<p>This is becoming a bigger challenge for merchants because they need to connect to thousands of nodes in the hope of spotting double-spends.</p>

<p>The <a href="https://github.com/bitcoinxt/bitcoinxt/releases" rel="nofollow">Bitcoin XT</a> patch set is an early release of functions that are proposed for Bitcoin Core.  It does relay double spends, and notifies the wallet of double-spend attempts that affect it.</p>

CC BY-SA 3.0
2015-02-28T18:12:44.910Z
2015-02-28T18:12:44.910Z
Tom Harding
<p><a href="http://blockchain.info/tx-index/242749/36080ed09eeb0fd66ad5dc8a909a49e364f55ff860c12327c2ff9fbe4a9dde77">http://blockchain.info/tx-index/242749/36080ed09eeb0fd66ad5dc8a909a49e364f55ff860c12327c2ff9fbe4a9dde77</a></p>

<blockquote>
  <p>Warning! this transaction is a double spend. You should be extremely
  careful when trusting any transactions to/from this sender.</p>
</blockquote>

<doublespend>
How does blockchain.info detect double spends?
ripper234
# How does blockchain.info detect double spends?

(Asked by: username_0 on 2012-03-02)

<http://blockchain.info/tx-index/242749/36080ed09eeb0fd66ad5dc8a909a49e364f55ff860c12327c2ff9fbe4a9dde77>

> Warning! this transaction is a double spend. You should be extremely
> careful when trusting any transactions to/from this sender.

**Tags:** doublespend


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## 2 answers

### Answer by username_1 on 2012-03-02. Score: 5

All valid transactions are relayed through the network. A client does not know which transactions will be a part of a block, so it stores and relays both double-spent transactions. If you regularly poll your bitcoind, you can see every transaction before it is put in a block. Then it is just a matter of checking txin data for duplicate inputs.


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### Answer by username_2 on 2015-02-28. Score: 3

The bitcoin network does NOT relay unconfirmed transactions that double spend other unconfirmed transactions, so neither blockchain.info not anyone else sees most unconfirmed double-spends. Nodes do not store two conflicting spends - they store only the first one they see.

This is becoming a bigger challenge for merchants because they need to connect to thousands of nodes in the hope of spotting double-spends.

The [Bitcoin XT](https://github.com/bitcoinxt/bitcoinxt/releases) patch set is an early release of functions that are proposed for Bitcoin Core. It does relay double spends, and notifies the wallet of double-spend attempts that affect it.


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