Is it possible to have two transaction IDs that are exactly the same?

[TeraPool]

Or would that be considered a collision of some sort? What exactly is a transaction id? A hash of some sort I presume?

I couldn't find much about transaction ids in particular on the wiki.

How would the bitcoin client treat two separate transactions with identical transaction ids?

[josell]

Yes, it is possible... But is very very very unprobable. They are more possible directions that atoms on the hole earth, but im not really sure.

[TeraPool]

Thanks for that.

That's what I thought.

Since blockexplorer allows you to search by transaction id... you should not be able to have a duplicate one I would think.

[theymos]

Transaction IDs are SHA-256 hashes of transactions. It's very safe to use them as unique IDs, and it's a waste of time to try to detect collisions.

It is possible (though very rare) for the exact same transaction to appear twice in the block chain, however.

[davux]

It is possible (though very rare) for the exact same transaction to appear twice in the block chain, however.

Can you please explain under what circumstances this could happen? I couldn't find anything.

[Maged]

It is possible (though very rare) for the exact same transaction to appear twice in the block chain, however.

Can you please explain under what circumstances this could happen? I couldn't find anything.

See blocks 91812 and 91842 for one example.

[JoelKatz]

You can maliciously, or stupidly, create two transactions with the same ID. The easiest way is to mine two blocks and include in each one only a single transaction -- the coinbase transaction, with the same payout address, same payout amount, and the same coinbase.