I was having this issue a lot.

To fix it I had to change the checkout submit button so that:
a) the button would dissapear after being hit (so they couldn't hit it 2,3,4 times)
b) an animated gif appeared (so they would realize the page was still working rather than hitting back, then returning to the checkout and hitting submit again)

Some combination of these worked for me:
https://www.zen-cart.com/showthread....76#post1317476
https://www.zen-cart.com/showthread....00#post1314200

That said, when I was doing a quick review of Zen-Cart 1.5.6c, I noticed a new file includes\modules\pages\checkout_confirmation\jscript_double_submit.php that reads as though it is meant to prevent this exact issue.