Z-tight has already answered you. I called them in the past on this forum 'pay 2 to get 4 scam'. It has been a old means scammers used to scam people.
Then I chatted on the event website chat, the person on the other end checked my transaction id and told me that their system decided that it was coming from a suspicious site, because of that it did not generate the expected send back to me for a double amount.
He will continue to look for something as an excuse for you to send more money. If you send the coins again, he will look for another means to ask you to send another one again and again.