In the case of team games, I have seen this happening if some players pick a color and others go random, especially if someone picks blue (other than player 1). I think player 2 picked blue in this case, same as in the example of @WiwiClementine. I was player 8 in the lobby and picked cyan, some others went random.
Until now, I have only seen the duplication happening with blue or green (or both, as in the example above). Sometimes only green gets duplicated.