Solo or Duo Queue for placements?

I don't think it's proven since mmr is hidden and it's really hard to tell losses due to opposing team being on average higher skill due to higher mmr, and losses due to opposing team being higher skill despite having comparable mmr.

My limited experience with duoing make me believe there is such an effect and it seems plausible to me (as after all, coordination is definitely an advantage over random soloers, so if you were matched with equal mmr people then it wouldn't be fair to them), but in the absence of a riot post confirming or denying it it's quite possible that it's just an illusion.

If you know you're comparable skill and you have duo synergy (e.g. bot lane duo or jg+anything) then of course you can duo for them if you want, I just feel like theres enough potential hiccups with the matchmaking for duos that going solo feels safer, especially if theres no special synergy with your duo partner to justify it. (Although in duo lane defence, that's one less person you need to worry about intentionally feeding or going afk [unless you have a very poor choice in duo partners].)