The "Show this canvas once per page" blog setting is a checkbox option found under the "canvas" tab in the properties panel on the right for any canvas in your blog template pages.
This setting works for all types of blog template pages. The purpose of this setting is to display content that will show on every blog page in your post list but will only show that canvas once. You might use this setting for a featured post canvas at the top of your blog page which shows on each subsequent page of older posts the reader navigates to.
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