What problem does this address?

Right now, is quite easy to end up with duplications on Classic Menus. Each time you click on 'Import from Classic Menu', a new block navigation menu is created, with no indication of the origin of that menu.

I did some experiments on #43580 with local storage, but, I think is not the best way to go, as the reference of menus imported would be done by user and browser, creating inconsistencies in any workflow.

What is your proposed solution?

If #42809 is landed, I think I can use the slug as a reference for both classic menus and nav menus to determine which ones have been already imported, and then apply this design approach (kudos to @javierarce here)