title: Reducer is adding items in the store out of nowhere
body: <p>So I am trying to add a cart feauture to my React-redux site and I got stuck on a very weird occurance. So this is what I get from the payload of the action for example:</p>
  <pre><code>{
      info: 'Fjallraven - Foldsack No. 1 Backpack, Fits 15 Laptops',
      id: 1,
      price: 109.95,
      image: 'https://fakestoreapi.com/img/81fPKd-2AYL._AC_SL1500_.jpg',
      count: 5,
      totalPrice: 549.75
   }
  </code></pre>
  <p>So what Im trying to do is, when an item with the same id as this one is trying to be added, to not add it, but to increase the count of the item with the same id that already exists in the cart:</p>
  <pre><code>const index = state.currentCart.findIndex((x) =&gt; x.id === id);
  return {
            ...state,
            currentCart: [
              ...state.currentCart,
              state.currentCart[index].count += 1,
               (state.currentCart[index].totalPrice =
                state.currentCart[index].price * state.currentCart[index].count), 
            ],
          };
  </code></pre>
  <p>The count itself is increased, but there is something really strange happening at the same time.
  The total price of the product and its count are also added as elements of the currentCart array, when the only thing that should happen is to update the count of the cart item with the id from the payload,
  this is what happens to the currentCart array when this action is fired:</p>
  <pre><code>currentCart: [
      {
        info: 'Fjallraven - Foldsack No. 1 Backpack, Fits 15 Laptops',
        id: 1,
        price: 109.95,
        image: 'https://fakestoreapi.com/img/81fPKd-2AYL._AC_SL1500_.jpg',
        count: 6,
        totalPrice: 659.7
      },
      2,
      219.9,
      3,
      329.85,
      4,
      439.8,
      5,
      549.75,
      6,
      659.7
    ]
  }
  </code></pre>
  <p>I am sure I am not mutating the state right, thank you in advance!</p>