There is already %player% but can you also add %hitplayer%.
If you hit someone with an item, the player that got hit his name wil be replaced with %hitplayer%.

I am holding an item with the command "duel %hitplayer%" and when I hit someone with that item his name will be replaced with %hitplayer%

For example:
When I hit Bob with an itemjoin item it will perform the command /duel Bob.

Sorry for my bad english.

If you could re-state this a little better like, go more in detail please. Its a little hard for me to understand.

Updated, I hope you understand it now.

This has been added as of ItemJoin v3.5 and it has been released!

You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=107086