Please let me point out first that our demo import system will not import content over and over. It's mentioned in article about our Demo: https://theme.co/apex/forum/t/setup-demo-content/56. I've also just tested to see if there's an issue with our demo import in my test site and I see no problem with duplication.
It's possible however that there's a unique or custom configuration in your WordPress install and/or web host that disrupts the process of our demo import thus duplicating the imported pages and posts. This does not happen in most web hosts.
When I imported the Agency demo and opened up the browser's console, I saw an admin-ajax error.
During the time of the demo installation, that error went from 2 to 18.
There are a lot of factors that cause Admin Ajax issues. To give you an idea, please read this third-party article: https://kinsta.com/blog/admin-ajax/
I'd first recommend that you first backup your site and try resetting WordPress or start building your site again from a clean WordPress install without any plugin but Cornerstone. If the import issue persists, I'd recommend that you try out other web hosts as we regretfully do not have control over web hosts settings.