Hodor
King Hodor of House Hodor
If you're up for game breaking (I'm assuming so since you want multiple Jarrin root to make uber mega poisons), here is a way to duplicate it:
Have a companion
Go to a place like say, Whiterun, right outside Breezehome
Drop the Jarrin root and then tell your companion to pick it up
You probably see where I'm going with this, if not, I'll continue:
Go/fast travel to the stables outside Whiterun, then travel or fast travel back inside the city.
There should be.. Could it be? *gasp* Another Jarrin Root?
???
Profit.
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However! Sometimes this doesn't always work. I duplicated my Jarrin root at the Dawnstar Sanctuary (end of DB questline) at the main door. Worked wonders. Just go from place to place and try it. I'm sure you'll eventually get an area that allows duplicating. Happy... uhm... Duplicating!
EDIT: If it's immersion breaking or lore breaking too much for you... pretend you made a special growing potion or spell that duplicates things...
I will try this thanks.