Ok, I just went throught the tut myself I have an idea what might have happened. When you click on "Duplicate Selected" the first time, it creates another frame, and both frames are selected, which means that when you again click Duplicate Selected, it's duplicating 2 frames, making 4 total.
Now, if you did this 15 times, you would get 2, 4, 8, 16, 32, 64, 128, etc etc until at 15 times I could imagine it getting upwards of a couple thousand!
What you can do is, first stretch out the window so that you can see more than one frame (it's easier to visualize the multiple frames that way). Also, look at the numbers underneath each frame. It'll say something like "F:1 D:10". The F:1 means that that is frame #1. The D:10 means the timing (duration) of that frame is 10.
So just duplicate until you see F:15 on the farthest right frame.
Hope this helps, let me know if you still have trouble!
CJ