Hello Richard,
just go on your backend -> Pages. Here you can view, select and delete your duplicated pages.

If you need more support on this I would be happy to answer you.

All the best,
Andrea

Hi,
the thing is that every time you import the demo, the contents are created again, and the items are added to the menu again.
if youw ant to completely “format” the website (removing also any content already exisgint, old posts and so on) you can use a plugin called WP reset, and then start over again.

Warning: this will erase everything, brining your WordPress to the same state it was once installed.

The duplicated menu items you are mentioning can instead be managed in Appearance > Menu, where you can delete all the items and create new ones, or delete the entire menu, create your own, and in Menu Locations assign it to the main menu location.