Duel mode must not have been on,
to know whether it's on or not, the
server will say that it has taken tc
from your account.. If you lose, it
would automatically send it to the
winner..
Seeing as you do not have a duel in
your transaction history, I'll guess
that duel mode wasn't on..
If you want to still pay the other
player, then do as LeB3ast said.