I can see how this might happen if there’s some JavaScript errors in the theme with AJAX carts. We can cover that shortly but what has me interested is this comment you made:
“When a customer tries to remove the extra item, it upsets the whole cart”
What does “upset” mean in this case? Something breaks? What do you see? Any context here may give some clues.
We’re having the same issue here. The customer informed us that they put 1 item in the cart and it turned into 4 in the cart. They didn’t realize until after they submitted the order. I tested the site and added things to the cart with no problem. The only “glitch” I had was after adding a couple items into my cart, I selected the “Buy It Now” option and it completely cleared my cart.
Any solutions? We’re using the free Dawn version 2.4.0 template.