Upon Project Import, clips in Timeline not linking to clips in Project
Upon importing one project into another, if "Allow Duplicate Media" is NOT selected, the program SHOULD see what media is in use in the timelines imported, and if already in project, link the timeline clips to that media.
I just discovered an instance in which Premiere does NOT create a duplicate clip, however, it doesn't actually link the clip in the timeline to the clip in the project panel - as you can see in this screencap, the clip name AS8A2568 is in the timeline, but the "Video Usage" is a 0 in the project panel, and "reveal in project" is unenabled in the timeline right-click. There is not another instance of this clip in my project. So, this looks like a bug where Premiere isn't properly linking the clips in this timeline (which was imported) to the media in the project I was importing it into.
Strange, though, that it didn't create duplicate media on import.
RYAN PATCH commented