DIAGNOSIS The duplexer (the internal mechanism that flips the paper) has guide plates or rollers that are either worn or have a buildup of paper dust and toner, causing the paper to skew and jam on the turn.
FIX Power off and unplug. Open the rear cover of the printer. You'll see the green plastic duplex paper path. Inspect it for any obvious obstructions or torn pieces of paper. Use a can of compressed air to blow out any dust from the entire path. Then, take a slightly damp cloth and wipe the green guide plates to remove any residue that might be causing friction.
IF THAT DIDN'T WORK Try printing on a different paper type or weight. The duplex unit on this model can be finicky with lighter-weight or non-standard paper. Switch to a fresh ream of standard 20lb or 24lb paper and see if the jams stop.
HEADS UP Avoid using paper that's been through the printer once already (like a misprint you're re-using). The heat from the fuser can slightly warp the paper, making it much more likely to jam in the tighter turns of the duplex unit.