2.0 tracks probably have better compatibility, for example my old Panasonic player outputs 1.0 PCM as 5.1 with only the center channel active, which works but is kind of weird.
That’s probably the reason but are all 2.0 tracks always a duplicate?
If the source was a mono film, they should be, but there are quite a few films that were mono in theaters but remixed to stereo for home video: https://www.hometheaterforum.com/community/threads/the-ultimate-list-of-mono-to-stereo-remixed-soundtracks.339481/
2.0 for stuff like Dolby Stereo shouldn’t be duplicated, obviously.