With Firefox you can determine if you have already bookmarked a URL by the presence/absence of the star at the right end of the URL. Clicking the star will bookmark a URL if not already bookmarked.
- If present it is NOT bookmarked.
- If absent it is bookmarked.
If it is bookmarked and you want another copy you should be able to select Bookmarks/Show All Bookmarks.
Then Enter something in the search box (upper right corner) to locate the existing bookmark.
Right click and select Copy.
Navigate to the folder you want to place it and right click - select Paste.
Does this not work for you?
Then to locate and remove the dups I dont want, I use this addon although it has not been updated since 2009 - but still works fine and I am running Fx 26.0a1 Nightly. This will list all the duplicates and where they exist. You select which one(s) you dont want and delete them.
https://addons.mozilla.org/en-US/firefox/addon/bookmark-duplicate-detector/There are other Bookmark Duplicate Cleaners if you want something more current.