How to double lenght of corrupted MP4 video?

Need help editing a corrupted MP4 file.

I don't know how to identify the header/bottom on a hex editor.

I used what I believe to be the header of a working file and pasted on the corrupted one. Now I can play the video, but it's all distorted.

I believe repairing it manually without having any experience will be almost impossible, so I googled it and found "hd video recover utility", but it's not free and the trial only recovers 50% of the video.

here's my question: how do I edit the hex files so I double my video lenght or make it play twice in the same file? (50% of the edited video would be my full video)

open with notepad

ffmpeg

On *nix:
cat video.mp4 video.mp4 > double_video.mp4
On WinDOS:
copy /b video.mp4+video.mp4 double_video.mp4

thanks for the copy command. it worked just as expected!