I"m not sure if it’s an error or I’m just missing something but 根本 (https://www.wanikani.com/vocabulary/根本) has this description:
The root origin is the root of something. Possibly even more root than root, but we’ll stick with root.
Shouldn’t there be “more origin than root”? I checked the dictionary and “根本” uses mostly something related to “source” or “origin” so it feels like it’s an error but I’m not native english so maybe is some funny idiom or whatnot that I just don’t get 
It’s just trying to be funny, doesn’t really mean anything. WK being a bit silly. Possibly also trying to help lock in the meaning for you by repeating (as WK sometimes does). Not necessarily an error, just maybe a tad bit confusing depending on how you read it. Although the next part of the description reads:
根本 generally refers to the root cause or fundamental basis of something, like “the root of the problem” or “the fundamental principle.”
yeah, it may be. I based it on the polish dictionary (I figured out that actually putting my native polish words in WK helps a lot with retention, despite being rather good with english – seems there is still some thing barrier :D). Alas, it lists:
hence my conclussion that considered both “origin” and “root” building blocks they were suggesting that both “origin” and “root” could work (with the former “possibly being more” [relevant]) but they opted to go with “root”
they’re trying to go for “more x than x,” as in “very x,” rather than “more like x than y”
i can certainly see the confusion, even for a native english speaker
if “origin” feels “very root” to you, then go for it
what would be a polish translation for “fundamental”? 
podstawowy (basic/essential/primary), fundamentalny (/fun.da.mɛnˈtal.nɘ/, yes, we are very creative 
but it comes from latin so there’s that: fundamentalny - Wiktionary, the free dictionary ), zasadniczy (principal/elementary)