Watchtower uses idea of watch folders — pre-defined folders that constantly look for any new content that is added to them.
If new content is found, it is automatically added to Premiere Pro/After Effects.
Watchtower compares file paths of already imported media and incoming media to determine if file is not in the project and should be imported.
sometimes file could have multiple file paths,
e.g. mapped drive path vs UNC path on Windows:
Z:\Media\file.mov vs \\Server\Media\file.mov
Watchtower considers these files different (same way as Premiere Pro & After Effect does). By default Watchtower will always use UNC path for import.
If you manually imported file via mapped drive path, Watchtower will import it again, but with UNC path.