This bug occurs when trying to pick a champ. It has no pattern of particular champs it affect; it is random.
~"Error selecting champion", "This champion has already been taken."
This message pops up, even though the champ has not been selected, and it still picks that champ anyway.
Seems to occur every other game.

I can attest to this as well. Happens seemingly at random.

I've had the same issue for a couple of months now. It's not that big of a deal really because as soon as I click "ok" I still have the champion I wanted selected, I don't even have to click them again for it to work or anything. But I do get the message pretty often at random. It doesn't apply to any specific champ either.