From MozillaZine Knowledge Base
Duplicate bookmarks are bookmarks referring to the same URL.
Firefox 3 introduced the bookmarks database file places.sqlite, and a new method of bookmarking from a current page with the star on the location bar. You cannot use the bookmark this page using the star or "Ctrl+D" to create a duplicate as you would simply change the folder but have the same bookmark as the the preexisting bookmark.
A bookmark keyword has a one to one correspondence with a bookmark, so if you need two different keywords to refer to the same URL, you must create duplicate bookmarks each having it's own keyword. A keyword is an alias for a bookmark.
Another use for duplicate bookmarks would be to refer to a webpage from different folders related to different topics.
It is still possible in Firefox 3 to create and retain duplicate bookmarks, but you have fewer choices to create duplicate bookmarks than you would to simply create a bookmark the first time. Clicking on the star to create another bookmark will not work, nor will "Ctrl+D" which will bring up the same dialog.
Creating duplicate bookmarks
Some methods of creating duplicate bookmarks
(Optional) Changes can be made to any bookmark so that it becomes more meaningful to you. Right-click on the bookmark and working from the "Properties" menu item change items available: Name, URL, Keyword, Tags, Description. Only changes to URL itself will make the bookmark unique to the Star dialog.
"Esc" key can be used to terminate an unwanted dragging operation before completion
Not deleting a duplicate bookmark
Major cause of lots of duplicate bookmarks
The major cause of ending up with lots of duplicate bookmarks, is by importing a bookmarks.html file into your bookmarks, the imported bookmarks are copied intact to the bottom of you your "Bookmarks Menu" and that includes all folders including making duplicates of "Toolbar Menu", "Bookmarks Menu", "Unsorted bookmarks". There is a major distinction between importing bookmarks from an HTML file (additive), and of restoring bookmarks from a JSON file (complete replacement).
Removing the correct duplicate bookmark
You can remove a single bookmark from a bookmark folder such as in the Bookmarks Sidebar ("Ctrl+B") or from the Library List ("Ctrl+Shift+B") but you should look at the bookmark properties to see why you have a duplicate such as needed in two different folders, or for a different keyword. If you use a search in the sidebar for the url name you may see 5 bookmarks, you would need to right-click to check the properties. If you use a search in the Library List you can check columns such as parent folder ("Show Parent Folder" extension, see below), keyword, description to see why you are maintaining duplicates and to remove only the one(s) you don't want.