Copying frames in a movie clip object error

Hi to every one. I need help with a simple issue.

I have a movie clip object and afther first 20 frames I want to copy and reverse them . That is quite simple to do, but afther I copy  and/or reverse this frames on the same layer, the first frame in this reverse process remains the same with the first one, the rest of them are correct displayed in the correct order.

Need help please.