When you encounter imbalanced games of League, we generally chalk it down to three major culprits: snowballing, matchmaking uncertainty, and/or perception. On top of this, there could be personal factors that matchmaking cant see—lack of sleep, one too many Graggy Ices, that sorta thing.

Do you get less LP for duo queue?

No, your LP gain is based only on your own hidden MMR. That said, if you frequently queue with someone lower than you, your MMR may decrease quite a bit (depending on how often you win/lose games), so its likely your LP gain will slow down faster than if you were solo queuing.