The printer is over a year old, and about once a year you should be having it maintained. No matter how many pages you print.
Dust, Humidity and other factors, can cause issues no matter how many pages have been printed.
The fact that it jams on duplex printing, suggests an internal component is not functioning correctly. Either dirty, broken, or begining to fail is the cause of a duplex print to jam.
But, if this was an inexpensive printer, and you didnt anticipate having it maintained over the years.....you can do duplex printing.....in a MANUAL Proces......and really isnt too hard.
Instead of having the printer, do the page flip....you do it.
Say you have 10 pages (front and back) to print.
Pages 1,3,5,7,9,11, etc ect will print first.
Then you pop thise pages BACKN in the paper drawer.....and the printer prints to the back side. pages 2,4,6,8,10 etc etc
This keeps the printer in SIMPLEX mode.......yet still allows duplex printing....with just a little manual intervention from the user (you) to pop the pages back in the paper tray in the correct orientation.
Thats how i have to print duplex.....as my printer only prints to 1 side.
I know this isnt what you had in mind......but it is a work around, until you can have the printer serviced, or service the printer yourself. But i dont reccomend this AT ALL! There are so many moving parts and such, that if you dont know....you can easily cause more damage without even knowing it.......printers are delicate devices.