The duoing can have a negative effect on your mmr (and it can have a positive one depending). You gain mmr when you win games, and you lose mmr when you lose games, but how much mmr to gain/lose depends on the average mmr of your opponents.
If you duo with someone with a much lower mmr than you, then your teams average mmr will likely be lower than your individual mmr, and so the opposing teams average mmr will likely be lower than your individual mmr, and so your mmr will gain relatively little if you win, but lose somewhat more if you lose. (Of course, duoing with someone worse than you to drag down the opposing teams average mmr does mean that you're more likely to be able to carry and win, so it's not necessarily disadvantageous to duo with low mmr people.)
The reverse is true as well, if you duo with high mmr people, then if you win the resulting games you'll stand to gain more mmr than you would solo, but you'd also lose less when you lost.