if i try to add same item after adding another item it says undefiend to all the other items beside newly added one about ecommerce_sanity_stripe HOT 1 OPEN

if(checkProductInCart) {
      const updatedCartItems = cartItems.map((cartProduct) => {
        if(cartProduct._id === product._id) return {
          ...cartProduct,
          quantity: cartProduct.quantity + quantity
        }
      })
      setCartItems(updatedCartItems);
    }

    if (checkProductInCart) {
            // find the index
            index = cartItems.findIndex((item) => item._id === product._id)
            //shallow copy
            let newCartItems = [...cartItems]
            //update the item
            checkProductInCart.quantity += quantity
            //insert back into the same place
            newCartItems[index] = { ...checkProductInCart }
            //update
            setCartItems(newCartItems)
        }