Why can't I create a duplicate bookmark?
I often bookmark the same site in multiple bookmark folders and have never had trouble doing this in the past. Now, when I go to add a bookmark to a second folder, it deletes the bookmark out of the first folder that I bookmarked it in. Is this a bug?
Wšě wotmołwy (3)
No, it's not a bug; it's intentional. You can revert to the old behavior by installing the following add-on.
You don't really need to use that add-on. It is possible to drag'n'drop the webpage favicon image on the tab into either the bookmarks sidebar or the menu bar drop-down to save a new bookmark, the same way as it works when saving to the bookmarks toolbar.
You can copy a bookmark in the Bookmarks sidebar or the Library if you want multiple occurrences of the same bookmark.
If you bookmark an URL that already exists the you only modify the already existing version as you can notice by the already highlighted star on the location bar.