The nitty gritty details are better answered by actual members of the LB team, like @rob or @lucifer. What I know is what I pick up here at the forum.
As I understand it, listens are linked to the database using hash values based on the the artist name and track name. So as long as these two bits of information are identical, the listens will be linked to the same MBID.
If two users manually make the same link, it becomes available to other users. I do not know what happens with conflicting data.