Duel Mode? Are you talking about Co-op or Battle mode? Co-Op Mode can be activated on the file select screen by pressing X. Battle Mode is coming to the Wii U, but it is not out yet for any version of the game. Does any of that answer your question?

yes

Co-op can also be accessed through the Shovel Knight amiibo.

Or by a pressing the minus button on a new profile.

the duel, FIGHT mode

So, it is Battle Mode you are asking about. As I posted earlier, it's not in the game yet. There is no way to access it, and there won't be until then.

It won't come till King Knight's dlc story comes.