I've had a few users with this issue. It seems all of them are printing a HTML page from Internet Explorer. I tried this myself and it does happen. The one page file changes them for a two page duplex job. This is how the transaction looks:
1 page(s) of "612x792,Letter,Mono,Duplex,Collate"; 1 page(s) of "612x792,Letter,Blank,Duplex,Collate" billed at .16 cents total (8 cents a side)
When I do the same job in Firefox is does this:
1 page(s) of "612x792,Letter,Mono,Simplex,Collate" billed at .10 cents a page (simplex since there wasn't two pages)
My inital thought was it is IE vs Firefox rendering it. Any other ideas?