Right, okay. I cannot for the life of me figure out how to copy and paste a keyframe in the same property in the timeline. I had assumed that as in all other adobe programs, you would be able to option (alt) click and drag, duplicating the selected keyframe. But apparently this is not the case? I've worked around it by sliding around my keyframes so I can create a new one with the same properties, but there must be a better way to do this. I must be missing something.
Select the keyframe, use the Edit menu Copy command (or keyboard shortcut), go to the desired point in time, and paste the keyframe using the menu or keyboard command.
Of course, I must have had the wrong things selected last time I tried. Bother. Works, thanks, sorry about the silly question!