The cause of the smudging on the second side is due to the top edge paper
curl that the paper gets on the first pass. This curling is due to
humidity, type of paper used and the heat of fusing.
One work around is to print the second side by:
1) Use "Page Setup" to flip the image both vertically and horizontally
2) Use the Multipurpose Tray
3) Feed the trail edge first into the printer
By flipping the image vertically and horizontally in the "Page Setup"
menu, the original trailing edge of the page is fed into the printer rather
than the original leading edge.