I had 2 matches where I encountered 2 players playing the same character. One of the matches had 2 Jacks, which is pretty great by the way, and in another match I was Kait along with another person.
I checked the footage I recorded from those games and noticed that in both cases, the duplicate characters had a different character skin.
In the match with 2 Jacks, one had the default Jack skin whereas the other had the Halo Reach version.
In the other match with 2 Kaits, I had the gilded skin and the other person had the winter armor version.
I haven’t done any testing but this strongly suggests that you can somehow bypass the duplicate rule by using the same character with a different skin than the other person.