also, it kinda makes sense if you think about it: you always see the site anchor on ct, typically the sentinel, having to deal with massive hits all the time. duelists have the opposite role on offense, where you have to deal with the stack that is potentially onsite. both are in charge of getting kills and value despite kinda being 1vX positions, and so it makes sense in a way that duelists switch to sentinel.
on another note, there are certainly different types of duelists. some, like aspas or something, would be better on agents like kayo or gekko because they are still part of the site entry. it's part of their playstyle inherently to aim duel and bully their way onto the site with util and trades. usually, they will be with the pack and be able to entry/trade their way onto a site. especially on flex like kayo, which usually means going out first if no duelist. however, other duelists, like prime yay and demon1, rely on being the 2nd one in or taking isolated aim duels. demon1 needed jawgemo, yay needed victor, to take the space at first so they could just swing and get 1v1 duels while they were too busy dealing with vic and jawg. these types of duelist thrive in 1v1s, so playing sentinel, a role with lots of passive waiting and 1v1 lurk fights, would make sense. however, the other type, in my opinion, the more aggressive duelist will be better playing a flash entry or just straight up sticking to duelist