if you are pasting multiple frames from tween_end.fla into a single blank keyframe in tween_start.fla, then u will see stacked frames of copied frames. To avoid this, u should create same number of blank keyframes on multiple layers. Make sure layers are same in both tween_start.fla and tween_end.fla. Then do paste. It should work.
I have created the same number of keyframes and kept the layers the same as you suggested and it still doesn't work. What I don't understand about this problem is, how does the timeline know that what you are pasting into the keyframe is the new symbol that you have just created? There is no command that says Copy Symbol and when you paste into the keyframes you can only choose Insert Frame, Insert Keyframe, or Paste Frames so how do you get it to paste the symbol you just made? Thanks for your help anyway.
This was just a non-standard way of creating a tween that had worked once accidentally but never worked again. I went back to the tutorials and learnt the standard way of doing it and then it worked.