An entry from the moz_places db may have several referring entries
in the moz_bookmarks db, i.e. where moz_places.id = moz_bookmarks.fk.
One of these entries is the "main" entry, while the other ones are used
for tags. Only the main entry has a title, while the others have not.
The tag entries have the same type as the main entry, even the parents
have the same type (folder).
Another source for duplicate URLs are manually created bookmarks, e.g.
in different folders. These may have the same or different titles.
To remove these duplicates, merge all entries with the same URL. If a
URL has multiple entries, keep all with distinct titles, otherwise keep
at least one - a bookmark may have an empty title.
See also T9626