well if you try to scale keys and the snap to frame isn't on, i know you can take a look in the graph editor and you'll find that they actually aren't on the same frame, one will be on the frame (say frame 12) and one will be on like frame 12.3... if you zoom in enough, you can just manually select and change one of the keys to frame 13... if you're trying to scale keys for multiple objects, you can zoom not only in x and y in the graph editor, but also in just x or just y by holding shift while zooming. (alt + shift + moving left and right will squish the frames together, while alt + shift + moving up and down will squish the values together while leaving the frames where they are)

hope that helps.