"MediaDuration" and "MediaDuration (1)" are simply different tag keys for the same tag name. This happens whenever a duplicate tag name is encountered.

Duration should be the length of the Movie. If it has separate tracks, each track may have its own duration. The length of sequential tracks should add to the total movie length provided they run for the entire movie. I'm not entirely sure what MediaDuration is, and would have to consult the QuickTime or MP4 documentation to figure this out.

Sometimes it is useful to decode the file with -v, -v2 or -v3 to see the structure. This sometimes helps in understanding. - Phil