Yes it's always a new link, and the reasoning for this is here:
It should be possible to have multiple share links for a given note. For example: I share a note with one person, then the same note with a different person. I revoke the share for one person, but I sill want the other person to access the note.
So when a share link is created for a note, the API always returns a new link.
In my observation, this is not true, that's why I am asking. It happened frequently enough, so I wondered about the conditions.
Sharing a published note again yields the same URL (which is good in my opinion, so partners get the new version without having to informed about a new URL). As I edited the note since the last publishing, this fact alone cannot trigger a new URL.
Maybe there should be a trigger for a new URL with a switch to delete the old URL.