Is the client sending that info back to the server every time it requests more posts? You can do the same thing with pagination. Embed a list of post IDs into the next button.

There's potentially a difference if the server's sending repeat posts and you're doing client-side filtering of what to show next. But do any sites work that way?

(And of course these issues only exist when your list of results changes order. It the list merely grows then you can paginate with start=xxx)