1. What was the last duel you remember? 
2. Does anyone still actually duel or is it just used to threaten people who mug you?
3. Should duels make a big comeback to solve issues without spilling collateral blood or are they pointless in a criminal world where an uninformed attack often assures victory?
4. Should there be a semi-annual tournament where people can duel for a big credit-based prize? If yes, how would it work?  

There were just a few pretty huge duels that happened only a couple of weeks ago. One ended a war early, and another saw the loss of a Godfather-Chairmen and also ended a war early. That's extremely rare, but it happens.

There can be a tournament for anything if the players want it enough, but I can't see it working. Accounts with age and kills behind them, the types of people everyone would tune in to see duel, would almost never agree to dueling each other because of how many months it takes to build those sorts of characters. So you'd get one or two good duels with people who don't give a fuck and then no one else would sign up so it wouldn't be much of a tourney.