or duoing with someone in a higher league than you, i started in silver 4 and i got like +30 per game and then when in silver 3 got +28 per game cause he's in silver 1 lol.
Duoing with somebody in a higher league than you is generally frowned upon
After a certain point, the matchmaking system will match you with a team that has high odds of beating you
Example, somebody in silver duoing with somebody in platinum
It's a system put in place to help prevent elo boosting. Generally, you should only duo with somebody in your own tier, or one that you're not far in divisions from.