I have several browsers on my computer. Brave, Firefox, Chrome & Edge because not all browsers work on all websites. This problem happens with Edge also. I don’t use Chrome or Firefox as often so I’m not sure if the problem exits there too.
For some reason when I open Brave on multiple devices I end up with TONS of duplicate bookmarks on all the devices, including bookmarks I have deleted a long time ago. At least in Edge when this happens the duplicates are all at the bottom of the list, in Brave they are all randomly filled in within the folder. When you have 500+ bookmarks, in a variety of folders you can see how this can be a big problem.