Duplex mixed page sizes ( Letter & Legal)

add duplex with a document that contains different size media (letter & Legal etc). Currently if a 16 page document has the first 5 (odd number) pages as one media (letter) than the next media is legal then the duplex does not work and adds blank page at end of document, Problem is that the front side of this insert page may contain information from the previous record and the other side of the page contains a new record, mixing them up.