Some people have had a problem printing two-sided pages on a LaserWriter. The pages print fine on the first side, but when printing on the second side, the toner smears at the top and the bottom of the page. There is no problem when the paper is manually fed into the printer. This can happen even when you're not using special paper.
The probable cause of the smearing or smudging is the fact that the fuser rollers in Apple LaserWriter printers have a small amount of lubricant on them. The second pass may pick up some of this lubricant, causing the problem.
Your best bet is to experiment with other paper stock until you find one that is less sensitive to the minute amount of lubricant. Consider using paper that is specifically designed for use with photocopiers or laser printers. Definitely avoid Xerographic paper that is designed to have animage placed strictly on one side. For more details on papers, search the Tech Info Library under "James River", "Scotch", and "Avery".