ajax function calls twice – Jquery ajax

i have a created a function whereby one can increase and decrease the quantity of a product using the plus and minus button on click.the function works very well but it call the function twice.if i press the plus button in creases the number by 2..i haven’t understood where the bug might be as i want to call the function only one and increase/reduce it by one.here is my plus/minus code in the blade file

 <div class="def-number-input number-input safari_only mb-0 w-100">
                                <button  onclick="this.parentNode.querySelector('input[type=number]').stepDown()" class="itemupdate qtyminus" type="button" data-cartid="{{ $item['id'] }}">
                                    <i class="fa fa-minus" aria-hidden="true"></i></button>
                                    <input data-id={{ $item->id }} class="quantity" min="1" name="quantity[]" value="{{ $item['quantity'] }}" type="number">
                                <button onclick="this.parentNode.querySelector('input[type=number]').stepUp()" class="itemupdate qtyplus" type="button" data-cartid="{{ $item['id'] }}"><i class="fa fa-plus" aria-hidden="true"></i></button>
                            </div> 

here is my ajax function in the script file

$(document).on(‘click’,’.itemupdate’,function(){

if($(this).hasClass('qtyminus')){
    var quantity=$(this).next().val();
    // console.log("the quantity is ",quantity);
    new_qty=parseInt(quantity)-1;
    if(quantity<=1){
        alert("item must be greater or equal to 1");
        return false;
    }else{
        new_qty=parseInt(quantity)-1;
    }
}

if($(this).hasClass('qtyplus')){
    // console.log($(this).prev());
    var quantity=$(this).prev().val();
    new_qty=parseInt(quantity)+1;
}

var cartid=$(this).data('cartid');
$.ajax({
    data:{"cartid":cartid,"quantity":new_qty},
    url:'/updatecartitemquantity',
    type:'post',
    success:function(resp){
        $("#appendcartitems").html(resp.view);
    },error:function(){
        alert("error");
    }
})

})

here is the function in the controller

  public function updatecartitem(Request $request){
    $shippingcharges=shipping_charge::where('is_shipping',1)->get();
    if($request->ajax()){
        $data=$request->all();
                // increment/decrement cart quantity items
        Cart::where('id',$data['cartid'])->update(['quantity'=>$data['quantity']]);
        $cartitems=Cart::usercartitems();
        return response()->json
            (['view'=>(string)view::make('frontend.product.cartitems')->with(compact('cartitems','shippingcharges'))
    ]);
    }
}