I've always had the Wikipedia main page bookmarked in "Other Bookmarks" but have just tried adding it to Bookmarks Toolbar and had both. However I found if I added it to a third place it did indeed replace the one on the Toolbar. However if I go to Bookmarks > Manage Bookmarks I can copy it as many times as I like — or certainly four at which point I stopped testing.

You need to create a copy to have the same bookmark (URL) in multiple locations. You can't do this via the star in the address bar as that will edit the current bookmark. You can create a copy via the right-click context menu (Copy) and paste it elsewhere. You can drag the tab or padlock icon to a bookmarks folder in a Bookmarks menu and to the Bookmarks Toolbar or to the sidebar to create a copy in that location. You can possible edit the name afterward.