Can I join a duel that already started?

Yes, if you were invited to it. An invitation stays good after the duel has started, so you can accept it late.

If the duel has a wager, your gems are taken when you accept, the same amount everyone else put in.

The duel still ends at its own end time, whenever you joined it.

You cannot join a duel you were not invited to. Duels are private to the people invited by name.

If nobody ever accepts, the duel is cancelled 7 days after it was started.

Learn more: How does a duel work?