IE stores its favorites as separate files and folders on the hard drive and that makes it impossible to have multiple entries with the same name.
So you get the same behavior as with creating or saving files on the hard drive.

Firefox uses an SQLite database file and that allows to store as many copies of the same URL as you want although links are used and there is only one actual entry if it is an exact copy (a click on the highlighted star on the location bar reveals this).
You can clearly see this if the URL is in the location bar, but not when you bookmark a link.

If you want to replace or update a bookmark then you will have to edit that bookmark, either via the star in the location bar or in the Library or sidebar or bookmarks toolbar.

If I'm reading correctly, this is an OS thing and can't be changed, right?

I don't know if this extensions helps in any way in this case (maybe select warn for duplicates).

• Old Add Bookmark Behavior: https://addons.mozilla.org/firefox/addon/8479

Yeah, it seems like that's the only option. It will probably be much more convenient than what I was initially looking for. I really appreciate the help. Thank you.