Ah, here we are then. If it's the big rewards you're after it definitely IS worth scanning duplicate plants on different bodies in the same system, because of the big "First to Log" bonus you get for each one. Here's two Tussocks samples of the same variety on two different moons in the same system I just got:
View attachment 344377
Even if you don't get a Codex for either, all that does is show your name in the Codex as a "First Reported by" for other commanders to view with their own logins. (You just see a blue "Confirmed" with your own login. )
You'll get some money for Codex but it's peanuts compared with the "First to Log" bonus you get for Vista submissions. I can't submit my own Codex until I arrive at a station somewhere (probably in the bubble) but my transactions screen still only shows a total of about 1.5 million so far and that's after I left the bubble, went across Inner Orion Spur and Hawking''s Gap and into Norma Expanse, whereas my Vista rewards so far since I left the bubble total about 4 billion.
I don't bother scanning duplicates. The time you spent doing that you could have moved on and scanned more originals that are worth 3 to 5 times that amount.