Duel of Champions was recently released on Steam and it is now much faster to load and upgrade than the previous stand-alone version. With this release they’ve also included 120 Steam Achievements that are tied to the achievements you can win in-game so that you can show them off directly through your Steam account publicly.
Unfortunately for the time being these achievements don’t seem to be syncing correctly with Steam. We’ve installed Duel of Champions through it and, even though we’ve already earned a few achievements in-game, they don’t show up as having been successfully accomplished on our Steam account.
There are also people complaining that even on freshly created accounts they can’t unlock the achievements so it seems that there’s no workaround and the only thing left to do is to wait for the Steam developers to fix it asap. If you want to help getting this fixed you can post on the several DoC threads being created on the Steam forums’ specific page and report this issue (although they’re probably already aware of it by now).
EDIT: The achievements bug was recently fixed and also they’ve included the Redeem Code button which had been missing on the Steam version of the game for a few weeks. (10.12.2013)