That is the purpose of Tags in bookmarks.
https://support.mozilla.org/en-US/kb/categorizing-bookmarks-make-them-easy-to-find

If you click-drag a bookmark into a bookmark folder you can save a duplicate bookmark, either into the bookmarks sidebar or the bookmarks dropdown on the Menu bar.

A post about how to duplicate a bookmark did come up recently: how can I save duplicate bookmarks with different titles? You may have seen that, since you say that you don't actually want to have to create physical duplicates.

The way the bookmarks are structured, each bookmark has one and only one folder. To my knowledge, there are no shortcuts/pointers. However, there are special folders which actually run searches. You've probably seen at least two of them, the "Recently Bookmarked" and "Recent Tags" folders. You might be able to add a particular tag to the bookmarks you want to duplicate, and then add queries to the other folders. It would be an extra level, though; the results would be on a flyout menu.

Ya not looking at using tags. Thought there would be some kind of add-on out there or grease monkey script out there that would allow this. Using tags still requires maintenance. It would take just as much time to copy and paste as it would to mark tags on each bookmark.

IMO, tags are great for people who can't remember where they put stuff. Being able to shortcut a folder would be the ideal way to eliminate duplicates and ensure both folders were exactly the same.

It blows my mind that nobody has created anything like this yet. Especially how versatile Firefox is.