Bitcoin Core 0.13 -- jsonrpc interface -- conflicted transactions
If you call listtransaction, you should get a JSON array called walletconflicts.

If your transaction conflicts with another transaction in your mempool or with a transaction in a block, this walletconflicts array is filled with n transactions-id's that points to the tx that spends the same, conflicting inputs.

walletconflicts is also present in gettransaction