PRINTER BEGAN PRINTING FRONT TO BACK WHEN IT WAS PREVIOUSLY JUST PRINTING ONE PAGE PER PAPER

HERE I AM MINDING MY OWN BUSINESS. MY HP PHOTOSMART 7520 IS NOW ONLY PRINTING FRONT TO BACK WHEN PREVIOUSLY IT WAS PRINTING ONE PAGE PERSHEET. THE MACHINE IS ONLY A FEW MONTHS OLD AND NOT BEATEN UP. HOW CAN I FIX THE MACHINE SO THAT IT ONLY PRINTS ONE PAGE PER SHEET? NO INFO ON THIS IN USER MANUAL AND I AM EXPECTED TO SPEND $24.OO WITH HP TO GET THE ANSWER. AMAZING.....