Korean has two complete sets of numbers. Every learner hears this and thinks: twice the memorising, twice the chances to be wrong.
It is not twice the work, because the two sets do not compete. Each one owns a territory, the border between them is sharp, and one question puts you on the right side of it almost every time.
1 · Where the two sets came from
The native set is original Korean, the words Koreans were using before Chinese writing arrived. The Sino set came in with Chinese characters and settled into the written, measured, official half of the language.
Most languages have a thin version of this. Alongside your ordinary two and three you probably also have a borrowed pair living inside words like duet · trio · quartet, or decade, octave, million. Those borrowed numbers turn up in technical and written vocabulary; the native ones do the everyday counting.
Korean did the same thing, and then kept both sets complete and out in the open instead of letting one shrink into a handful of learned words.