title: How to prevent adding same product to cart more then one time in magento
body: <p>I am new in magento development. I dont want user to add any product more then once to the cart.if he/she need to change the qty of the product then he/she need to change it from mycart page but add to cart button only allow once to add product, after that when he/she clicks on add to cart button it must say "it is already added to the cart if you want to change the quantity then please go to mycart".</p>
  
  <p>for example you can see www.flipkart.com.</p>