If you are sometimes getting unique items in the cart and not other times it seems that you may have a problem with a recordset that is being used by the add to cart server behavior. In order to further troubleshoot what is going on you will need to get this to a point where it can be reproduced with a particular set of steps. This will be the key to figuring out what is occurring. Try running some test after closing your browser and adding some items in a particular order then. You should be trying to keep track of what you are doing in between your tests and when you see the result again you will need to make it happen consistently. Otherwise it will be very difficult to determine what is going on.
