MultipleObjectsReturned at /add-to-cart/ get（）返回了多个Cart --它返回了2个！

@login_required
def add_to_cart(request):
    user = request.user
    product_id = request.GET.get('prod_id')
    if Cart.objects.filter(product=product_id).exists():
        prod_id = request.GET['prod_id']
    try:
        product = Cart.objects.get(product=prod_id)
    except Cart.DoesNotExist:
        product = None
        c = Cart.objects.get(Q(product=prod_id) & Q(user=request.user))
        c.quantity += 1
        c.save()
        user = request.user
        cart = Cart.objects.filter(user=user)
        amount = 0        
        for p in cart:
            value = p.quantity * p.product.discounted_price
            amount = amount + value
        totalamount = amount + 300
        print(prod_id)
        data = {
            'quantity': c.quantity,
            'amount': amount,
            'totalamount': totalamount
        }
        return redirect('/cart')
    else:
        pass
        product = Product.objects.get(id=product_id)
        Cart(user=user, product=product).save()
        return redirect('/cart')

我试过了，请帮帮我。如果产品在另一个购物篮中，则不希望将其添加到用户。