JSON-RPC to detect double-spent TXs How can I be positive that one of the incoming payments to my bitcoin-core was double-spent after it was confirmed once by the network, through the JSON-RPC interface?

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How do I detect a double-spend?
Accepting 0-confirmation transactions and detecting double-spends with open source solution
https://bitcointalk.org/index.php?topic=327767.0 (GHash.IO and double-spending against BetCoin Dice)
If your node received the original spend, the listunspent RPC will display the transaction with "-1" confirmations, as will any other RPCs that would normally list the transaction.  The source code calls this conflicted: "not in the mempool and not in the block chain."  This feature was first included in 0.9.0.

If your node never received the original spend (perhaps because it was offline at the time) or the now-stale block that contained it, it doesn't have any way to know about that original spend, so there's no information you can get from an RPC.