Punishing Duos

At the beginning of the season, when I was refusing to play ranked with my friends, I had a 55% win rate and was climbing very fast. After I hit gold 2 and figured I could probably just cruise into plat, I started duoing with one of my silver friends to help him out a little bit.

Big mistake.

Every time I duo, my mmr drops horrendously low, regardless of a win or a loss. I went from playing in games with plats and low diamonds to playing with low silvers almost every game I queued up for, even when I wasn't duoing. I stopped playing with them again and it took about 2 weeks to climb back up, and then I made the mistake of duoing again. It doesn't matter how few games I play, my mmr seems to get set artificially low even after I'm done duoing and back to solo. I went from getting ~23 lp a game to ~15, and went from losing ~12 to ~20.

What I want to know is why I'm being punished for playing with a lower ranked friend even after I've finished playing with him.

Side note: This has particular post has nothing to do with the whole "dynamic system putting solo players into 4 man queues" thing, although that's a huge problem that I face as well, but its well documented already.