[sciurius]

Two users with separate libraries may import the same file off Dropbox let's say, then add their own annotations and other fields. If they merge their libraries, my logic should handle the fact that they both created the same song (assuming they used the same title), and merge it.

I beg to differ. Even though they both used the same file to start with, each user created a unique song. I personally doubt it has any use to merge. For example, if it is a chordpro file, annotations are relative to the rendering of song lines, and that depends on individual settings like text size and such.
If one user replaces the file by a totally different one, would you still consider it to be the same song?

One possible way to handle this issue would be to add a new song number field that has to be unique.

UUID?
If you make it an editable property the user has ultimate control over whether two songs are to be taken ‘the same’, or not.