Laserjet 1320 pulls two sheets of paper (one after the other) when printing a single page. It does this when printing a test page.
As another example, if I hold the green button til it prints the configuration page (which is 2 pages), then it sends out 4 sheets total: the first page (followed by a blank page), then the second printed page followed by another blank page.
According to the configuration page, the total page count doesn't include the blank pages that follow each of the printed pages.
Oddly, I have several of these Laserjet 1320 printers that do the exact same thing. I have replaced the pickup rollers for both (which were worn enough to be smooth) and have cleaned the separation pads with alcohol.
Is there a sensor that should tell the printer to stop after it has picked up the sheet of paper?