DIAGNOSIS That's called ghosting, and on this printer, it's almost always caused by a worn or dirty fuser unit—the hot rollers that melt the toner onto the paper. The old image is getting thermally re-transferred onto the new page.
FIX The first step is to try a different type of paper. Heavier paper can sometimes reduce ghosting temporarily. If that doesn't help, you need to replace the fuser. It's a user-replaceable part on this model. Order part number CF410A (the fuser kit). Power off and unplug the printer. Open the rear door. Locate the blue fuser release lever on the left side, pull it towards you, and then pull the fuser assembly straight out. Slide the new one in, push until it seats, and push the blue lever back. It's a straightforward swap.
IF THAT DIDN'T WORK In rare cases, a defective toner cartridge (especially the black one) can cause ghosting. Try swapping in a known-good cartridge to test. Also, ensure you're using the correct paper type setting in your print driver for the media you're using.
HEADS UP The fuser is a consumable part with a lifespan (typically around 100,000 pages). Ghosting is its classic end-of-life symptom. Continuing to use a worn fuser won't damage other parts, but your print quality will keep degrading.