I haven't tried the Effaclar Duo, but my guess would be that the Effaclar K is more aimed at prevention.

Also, if you're anything like me, your diet is strongly related to your acne; it might be worth going to see a specialist who can give advice on what you should avoid. I break out every time I touch anything that has dairy in it, although unfortunately I love cheese and yoghurt too much to cut them from my diet completely, so I've resigned myself to the occasional relapse. :)