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I have the original CD version of the game, and double-jumping does not work when its running in a window. This for both the original installation and the 1.01 patch. Anyone have this problem with the GOG version?
Unfortunatly the GOG version still appears to have this problem.
It's because of running in a windowed mode. It's already been covered in this post in this forum:
Actually I managed to get double-jumping to work in windowed mode - all you have to do is switch from Direct3D to OpenGL!
Now I'm running in a 1440x1080 Window (my monitor doesn't support this res in full screen for whatever reason) and the game looks great!