>>18113906A rehash is when you take the exact same elements out of one data structure and put them into another, usually but not necessarily bigger structure. The order they're in can change but using an iterator you should be able to visit all the elements once in both cases.
The very fact that there's more Pokemon in-between games basically means that no generation is a re-hash of the other.
Now strictly speaking, Red is indeed a rehash of Blue as are every other group of two games released at once. Red is however not a rehash of Yellow and so on and so forth.
