Allow duplicating files/segments in when merging/concatenating multiple files

[markusd1984]

A nice to have option, which would enable to achieve repeated segments in a merged output without having to first duplicate the source segment files.

Better but likely more difficult to implement would be to duplicate segments within the timeline, which then also perhaps would need to support arranging segment placement within the timeline #596.

use case example #284 (comment)

[markusd1984]

A duplicate icon next to each imported file would be cool!

The issue in #634 may need to be solved though, perhaps an option to enable normal cut mode for exact transition between two files.

[mifi]

as suggested in #1256 - alternatively being able to add the same file many times