why do some units have double of the same frame

Ground units turn in fewer directions than air units. If you look, all of the units with doubled frames are ground.

huh, that sux
why does it have to be doubled?

So that the units have the proper number of frames. Just overwrite both frames (with the same image) when you edit the GRP and it will work fine.

They are only like that because they were lazily made, you don't have to put duplicates, it just means creating less renders.

They double up because iscript wants that many frames, and they don't have that many.

Thats not what i meant. Yes you require 17 frames for each frameset if the graphics turns, but you don't have to do the duplicate frames thing, you can just render all 17 directions. They probably only did it because the were lazy, its half the renders and its not that noticable in game if two different angles use the same rendered angle.