title: Magento - prevent adding the same sku twice or more
body: <p>I want to prevent users from adding the same product twice or more (in backend as well as frontend), whilst not restricting them from ordering as many items as they want.</p>
  
  <p>For example, given there's a product with the SKU <strong>ABCD123</strong> - how do I prevent the user from adding it twice into the cart? Instead allow the user to add it once and let them update the ordered qty instead?</p>
  
  <p>I had a look at this <a href="https://stackoverflow.com/questions/6545329/how-to-prevent-adding-same-product-to-cart-more-then-one-time-in-magento">How to prevent adding same product to cart more then one time in magento</a>; however the solution suggested refers to restricting total number of items in cart or updating every product with a certain inventory config. </p>
  
  <p>Is there a way to do this differently by modifying the cart code?</p>