This awesome mode was omitted from the HD versions for some strange reason. It exists in the original Snake Eater and Subsistence versions of the game and is unlocked after beating the game once.
In Duel, there is a 1st place score to beat in each encounter and beating said score in all encounters earns you a reward. For Duel's Special mode, the reward is the DPM uniform which provides excellent camouflage in forest environments, and also doubles your LIFE recovery rate.

You will probably notice that I'm not getting the 1st place in many encounters, but that is because I've already beaten the original 1st place score in every encounter prior to recording this video. I added a caption to each score screen showing you what the original score to beat is.

This was played on PC using the PCSX2 emulator.