…or the same track twice but one is a demo/single and the other is the album version - both titled the same but one is slightly different in length. I have a lot of these with Indie music.
It compares audio signatures generated by Traktor, which ensures that tracks are identical, even if they have different file names or id3 tags. As for the tracks on different releases, it depends whether they are digitally identical. For example if a track on a different release is slightly edited (for example faded out) it will be considered as a standalone track.