There is a little known Magento bug that can be hard to diagnose where every time a customer clicks "add to cart" the desired quantity is doubled. While there are some variants, this issue is generally only seen on Internet Explorer (version 8 and earlier) and older versions of Magento (older than 1.6).
If you see this behavior and don't want to upgrade your Magento installation there is a quick fix you can do yourself that will resolve the issue...
- Log on to your server via FTP or SSH and look for the file app/design/frontend/default/default/template/catalog/product/view/addtocart.phtml. Remember to replace "default/default/" with your particular template path.
- Look in the file for a piece of code that includes "productAddToCartForm.submit()"
- Modify that piece of code to look like "productAddToCartForm.submit(); return false;"
- Flush your Magento cache and the problem should be fixed!