How many time can I work on a single mp4 video file before the quality degrades?

This is really a two part question so please bear with me since I am not an expert on these things.

A couple of years ago, a friend of mine downloaded a 10 minute MP4 video from YouTube and gave it to me on a flash drive. I then used Windows Movie Maker (not the best program but its what I have) to brighten the video a little bit and then save the file in HD which is 1080p. I then converted the new HD video using RealPlayer converter so I can play it on my iPod/iPhone. After all was done, I deleted the original video that my friend gave me because since I did "my version" of that video, I didn't think I needed the original anymore

It wasn't until recently that I noticed that, in the middle of the video, were a couple of seconds that repeat three times. Now I want to use this video on Movie Maker to edit out those a couple of seconds, save it in HD, and convert it using Real Player converter. I've read that reusing a video file many times will eventually degrade the quality of the video so:

My first question is: Will "redoing" all this a third time further degrade the video?

and

My second question and this is somewhat unrelated: Will copying a video to a flash drive or copying to another folder degrade the video?

I would greatly appreciate the help.

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