When you print on a sheet that has been previously printed on, the toner rubs off of the page and onto the rollers inside the printer. Not only will the roller transfer toner ink to other print jobs but it will also push the toner into the gears and working parts of the printer. Toner is very fine dust and will eventually wear and jam the mechanisms inside the printer. To ensure the longevity of the printer, the lab has restricted the printing of documents on paper that has already been printed. Only the Dell printer located in the Sinclair computer lab has a duplexing unit that allows for double-sided printing.