Some people posting here are absolutely clueless. You're debating about how to implement a system that is already in place. Let me try to explain: each player has a regular level, and a duelling level. Your duelling level is a minimum of your regular level, but if you have stacked up on your duelling skills then your duelling level will be higher than your regular level, but never lower.
Now, when you challenge someone to a duel, the difference in duelling level is calculated. If the difference is too large (it's around 3-4 levels for a lvl 17 character), then the player cannot be challenged.
If the player is asleep, they cannot be challenged.
If you don't have enough energy, they cannot be challenged.
If you duel a lot, your motivation goes way down as with a regular job, so it's not as if you can just duel all the time.
If you duel someone much lower level than you, you get virtually no experience. If they have banked their cash, you don't even get cash from the win.
Usually all that happens if you are challenged, and you lose, is that you lose a bit of your cash. But this isn't a problem if you use that very handy feature called a bank. If you win you gain experience for free. It doesn't cost you energy to be the "victim" of the duel, even if you win.
Cliffs: If you stash your cash, being a duel "victim" has more positives than negatives.