Hi Giovanni,
When you decide to import another demo to the site you add its content to the already imported one. In other words, old data is not removed when the new demo is imported. Thus it is recommended to reset the WordPress installation and delete all the existing content – properties, bookings, posts, pages, media, etc. You may use the third-party plugin to reset WordPress installation.
If you wish to keep some of the existing data you need to delete the unnecessary content manually.
best regards,
John Davis