For paper weight specifications, see the reference guide that came with the printer.
Paper is loaded improperly
The printer might print on multiple sheets of paper at once if the paper is loaded too
far toward the back of the printer. Reload paper into the paper tray, pushing the
paper until you feel resistance.
HP Deskjet 6500 series printers
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The printer might print multiple sheets if different paper types are loaded in the
printer. For example, photo paper might be mixed with plain paper. Empty the paper
tray and load only the paper type that is appropriate for the document that you are
printing.