A low fee doesn't make it double spending, but 2 transactions were created, using the same input which is double spending. Blocktrail.com says that this transaction was a double spend:
https://www.blocktrail.com/BTC/tx/518cf0067c0356fe0d9b555111c30330e39cfc9935beb7b26aafd26f4907f72bThat input was also used for transaction e437a2c31a644bf0fd5e380aecb457df5b9250b189c50d81f6cbb385946774ae, which was the transaction that deposited into the account where you won. Pretty obvious double spend attempt. Wasn't expecting an attempt at casino.bitcoin.com, I was predicting that after Directbet/PD were hit, someone would try BitSler. Seems like a lot of people have been complaining that their cheating failed.
Don't ever buy BTC from that person again, if you actually bought btc.