Duchene is also a C, so even though he is the 15th LW, he also happens to be the 15th C. The cost of extending the 15th C is $6.7m per year which is higher than the LW cost. The extension rules state that we have to go with the "the higher salary of the two positions".
You could extend him now for the $6.7m, but that would only make sense if you think he is going to finish higher than the 15th C this year. Which he might because Duchene is a great player.