How can I delete one of them as it doesn't appear in Menu Manager anything related to mobile:

 

Does anyone confirm? Is this a bug?

It’s intentional. The main navigation links are clickable on desktop, but not on mobile, where they open the sub-menu. So they put the main menu link in the sub-menu as well — otherwise that link would be inaccessible on mobile. 

If the name is identical, it is weird, indeed. I usually make it a little less weird by using different menu names. Doesn’t change the functionality, but looks a little better. 

Yes, they are the same and both go to .../index.php?/support/

Just call the sub-menu link “tickets”, “ticket list“ or something like that. 

Otherwise, it’s something for 5.0. When they go with Mobile First as Rikki said, that behavior will probably improve in the future.