Limiting user from adding to cart multiple times
 

I want to stop the cart from receiving too many products. If I limit the quantity of a product on the details screen using the validation there, how do I stop a customer from just adding multiple times to the cart instead. Example:

Max quantity for product A is 1000

If user enters 2000 on product screen it gives them an error and says quanity is limited to 1000. But if the user goes back and adds 500 4 times it works.

Is there anyway to stop this in xcart automatically or is it a mod?