If the amount of BTC behind the door, after processing previous bets, is equal to or greater than the amount you send, then your bet is doubled!
I missed this game, but I am looking at this line and wondering why a player wouldn't send 5 btc to each address (or 5000 for that matter). It's guaranteed to have equal to or greater than the amount sent, and so in each case the bet would be doubled.
Even splitting in proportion the number of coins behind the door doesn't work (it's still a guaranteed win, although it's limited to 5 btc total).
You are lucky only one person played.