items are duplicated

32044053

As shown, How to remove duplicate items?
i can't even edit the main menu, since the server is not responding when i select it.

Air

That is bad, as it sound like issue in data base. Tried using one click demo data import?
http://apollo13.eu/docs/blame/#!/installing_demo_data

You can always return your wordpress to default https://wordpress.org/plugins/wordpress-reset/

With kind regards.