Open from your host's panel phpMyAdmin to edit your database.
Go to your xxxx_menu table. (xxxx is randomly generated, it's your db's prefix).
Sort by the Title column.
Scroll and search for entries that are like com_banners~, Banners~, Contacts~. The tilde ~ in the end is the duplicate.
Delete all the entries with a tilde (or two) in the end.

- Go to menu -> administrator
- Create a new administrator menu, without items
- Go to modules -> administrator
- Delete of modificate "administration menu"
- position -> menu
- menu to show - > "Your new menu"
- check menu -> NO
- Save