Galleries merging?

I am having a problem getting my galleries to display only on the pages in which I place them.

Every time I create a new gallery, all of the images appear on every page of my site. This happens even if I link the gallery to a specific page and it occurs before I even insert the gallery into the page. I only want to have that particular gallery on that particular page but they seem to merge somehow. As soon as I create a new gallery, all of images appear on my (static) home page as if they were included in the gallery
displayed on that page and the home page gallery images appear as part of the gallery on the new page.

This is not related to the theme I am using because I have tried switching themes several times and the problem persists.

I was given this suggestion
“This happens when page content or the gallery shortcode added via the Insert Gallery Window, has been copied and pasted into another page or post. When you add a gallery through the Insert Gallery Window, a gallery ID is registered into your site’s database. This ID is unique to that inserted gallery. So when you copy and paste this gallery shortcode into another page, when you update the gallery on that new page, it will automatically update the other gallery because it shares the same registered ID. To resolve this, on that new page, you’ll need to remove that shortcode and re-insert a new gallery on that page so it can receive it’s own unique ID”

I do not see any option in any of the editor windows eight for the page or the gallery that makes any reference to shortcode. How do I change the shortcode for a particular gallery or page?

Can anyone help me resolve this?
I am using the pro version of Nextgen and the “masonry” style gallery.

https://www.ads-software.com/plugins/nextgen-gallery/