MySQL On Duplicate Key Update

It is only allowing me to update resulting in a total of 2

Heres my Table

CREATE TABLE `cart` (
  `id` int(7) NOT NULL AUTO_INCREMENT,
  `User` int(7) DEFAULT NULL,
  `Product` varchar(100) DEFAULT NULL,
  `Quantity` int(7) DEFAULT NULL,
  UNIQUE KEY `id` (`id`),
  UNIQUE KEY `Quantity` (`Quantity`)
 )

Then, my code to insert the data is:

$a = '1'

query2 = "      INSERT INTO CART(User, Product,Quantity)
                VALUES
                ('$id','$model_number','$a')
              开发者_如何学C   ON DUPLICATE KEY UPDATE Quantity=Quantity+1";

It will work when I Add the data to the database but, upon my second attemp I get this error:

Duplicate entry '2' for key 'Quantity'

---

You probably don't want this line:

  UNIQUE KEY `Quantity` (`Quantity`)

That creates a unique constraint on the quantity field, which is why your second insert is failing. I can't think of any reason why you would want that.

P.S. If you remove that line, make sure to remove the comma (,) from the previous line.