Question: how do you tell when Duplicity (or anything else for that matter) procs? Is there some kind of visual or fly text that I'm missing?
Yep, there is no standard Raid UI for this game telling you "HEY, USE YOUR <Insert Skill Here> NOW"
Just keep an eye out on your frames and you will be fine.
Plus, tankasin's = the tankadin's of SWTOR. 7 Seconds of spamming and building threat, followed by 30 seconds of "Thrash, Thrash, Proc, Thrash, Thrash, Taunt, Thrash Thrash, etc..."