Question: "If that same user navigates to a different page, that has the same report embedded, does that user incur another session charge?"
My answer: "yes and no, it depends on how you code your application. An end user uses an app token and not an access key to view embedded reports. The main difference between an app token and an access key is your application provisions app tokens to the end user while azure provisions the access keys to your application. Because there exists a middle layer between azure and your end user (the middle layer being your application), azure doesn't have any knowledge of end users.
In your scenario, Azure knows that your application is requesting many sessions. Azure does not know your application's end user is requesting the same report multiple times. To resolve this, you have to make your application smart by managing each end user's app token efficiently.
Hope this addresses your question