LP Glitch?

This isn't the whole story. It can be the case, but isn't always.

A short-ish explanation would be that when you are in a match, your team's individual MMR's are averaged (in order to determine opponents to face you against, and how much LP you'll gain/lose). When you are Duo Queued with someone, since they usually have better coordination than average, those 2 people's MMR get a slight boost which in turn raises your team's average. According to your new higher MMR, you are expected to do better and will gain less LP for wins and lose more for losses than if you went alone and didn't have an adjusted MMR vs the same opponents. In other words; it's a handicap so you don't have an unfair advantage and easily win more than you should.

Feel free to duo queue with your friends. You'll still be better off than going alone, the handicap you're given isn't really that much. Keep in mind that the system attempts to place Duo Queue groups against each other so both sides have the same advantage (can't always happen, but it tries to). Duo queueing is more fun imo, anyway.

[{quoted}](name=Valkyrie Witch,realm=NA,application-id=LqLKtMpN,discussion-id=Io9LEHes,comment-id=00010001,timestamp=2015-04-26T23:29:20.444+0000)

That explains the low gain, not the discrepancy between how much LP loss was reported at the end-game screen and how much he actually lost. Unless I misread, he was saying he lost more than he was told he did.

You lose more LP for ff's games. And it could be a bug, but I've had games where I went into a series, won and it didn't even register that I was in a series. It didn't count for some reason, but the 2 after that did.. So there are some sort of wonky things that go on.