My transaction is marked as double spend. I don't remember when I spent it twice. How can I see what transaction used the same utxo?



You can find doublespends by looking at the addresses that the inputs were previously associated with. At least one of the "send-addresses" will show another transaction that spends one of in the inputs that your transaction is also claiming.

In this case, this is particularly easy, as there is only one input, and therefore only one previous transaction output to consider.

When you look at blockchain.info's page for the address 12sWrxRY7E7Nhmuyjbz4TtGE9jRewGqEZD, you'll notice that there are two competing transactions trying to spend the input.

In regard to which of the transactions may be confirmed:
Obviously, only one of them can ever confirm. In this case, their fee rates are 7.5 and 8.95 satoshi/byte, so likely neither will confirm. In other cases, it depends on the transaction selection strategy of the miner. AFAIK, first-seen remains the standard policy, i.e. nodes and/or miners accept, relay, and confirm the transaction that was first seen and shun the other as a doublespend. However, especially in a time with fee events and crazy mempool sizes, I'd expect more and more pools to adopt a replace-by-fee policy where they would prioritize the transaction with the higher fee. Not only do miners earn more that way, and the transaction will be up for confirmation quicker, but this also gives people a better chance to update urgent transactions that didn't have a sufficient fee the first time around.

Since this is the obvious direction transaction selection will progress, let me use this opportunity to reiterate that unconfirmed transactions are a payment promise and not a reliable payment. Accepting zero-confirmation transactions is a bet which becomes less reasonable by the block.




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    I've downvoted this answer because it doesn't explain and won't be useful to anyone else. Let's prioritize teaching people how to fish instead of providing fish. – Murch May 21 '17 at 2:50

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