I am currently silver 1 and duo with my bronze one friends a lot today. They both have silver mmr. I started at 1509 mmr according to op.gg but after winning like 10 and losing about 4 I now have 1294 and it says my mmr will continue to tank.
That's from a third party site. It basically guesses your MMR based on the average player rank in the last 10 games you played. If you're duoing with bronze players, your average rank in a game will be lower - so your MMR will appear to drop on third party sites.
It's fairly safe to assume that if you're winning more than you're losing, your MMR won't be dropping. The easiest way to tell is to check your LP gains and losses when playing solo. Gaining 20 per win and losing 20 per loss when solo is the normal amount. This can deviate by around 2. So if you're +- 18 to 22 LP from a solo game, you're where you should be.
Again, if you're duo'd with lower ranks you'll probably gain less per win (I'm honestly not sure but I'd guess) so you should check with a solo game if you really want to know.
