User send btc, merchant accepts btc after in OKPAY case , Six freaking hours of confirmations,
blockchain is reorged to a new chain without that specific transaction.
So those coins were spend twice. ie: double-spend
Once with OKPAY and after the reorg with someone else.
Actually, it's not a double-spend in this case. Because the spending inputs are included in one transaction, but there is no second transaction to different outputs the spending inputs are in. A reorg is
not transaction spending, because there is no spending initiated - the transaction inputs are still valid as if the first transaction never happened, in other words the first transaction was deleted. There was no second transaction at all, hence no double spend.
A double-spend would be if you submit a transaction to one miner, and then the same transaction but with different outputs to another miner and the two miners don't sync fast enough before a large number of nodes on both side have one of the different chain forks.
Reorganizing the blockchain to reverse a transaction is a fraud and it is possible to do it on btc's network.