user78: 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.


user21546: 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 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.

user323: 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.

78: <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>
 21546: <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>
 323: <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>
