Event fires once more for each click

Question

I was making a store application, in the application, when clicked on the product image, opens a modal indicating the quantity. This modal has a button submit to send the data in the database.

By clicking on submit the first time the event runs smoothly. The problem is that the second time you click happens as if I clicked 2 times, the third as if I clicked 3 times, and so on.

I searched and found a solution: unbind() or .off(), but I didn’t understand why it happened, when an event ends click should end, no?

In short, my doubts are these:

At the end of the event click the function continues to be executed?

If the .unbind() disables all events click or others, the bind serves to activate them? In other words, it does the same as the .trigger()?

I realized there’s a difference between:

$(document).on('click', "#div", function(){....}); and $("#div").click(function(){...});

I just didn’t understand which, but it seems that when using the second option, there will often be some error, so it is always better to use the first option, correct?

EDITED

As requested I will add the code:

<!-- TRIGGER --> 
<button class="botaopreco" onclick="addcart(<? echo $prod['id']; ?>)">
    Adicionar 
    <i class="fa fa-cart-plus" aria-hidden="true"></i>
</button>

Modal

<!--:::::::::: MODAL :::::::: -->
<div class="modal fade" id="myModal<? echo $contador; ?>" tabindex="-1" role="dialog" aria-labelledby="myLargeModalLabel">
    <div class="modal-dialog modal-lg">
        <div class="modal-content">

            <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
                                <h4 class="modal-title" id="myModalLabel">DETALHES</h4>
            </div>

            <div class="modal-body">
            <div class="imagem-modal">
                <img src="imagens/<?echo $prod['caminho_imagem']; ?>">
            </div>


            <div class="modal-info">

                <div class="info-title">
                 DETALHES
                </div>


                <div class="info-info">
                    <? echo $prod['detalhes']; ?>
                </div>

            </div>

            <br><br>

            <div class="modal-info">

                <div class="info-title-two">
                STOCK
                </div>

                <div class="info-info-two">
                <?
                    if($prod['stock'] == "EM STOCK")
                {
                    echo '<span id="stock'.$prod['id'].'">'.$prod['stock'].'</span>';
                    echo '<i style="color:#7CBB00;position:relative;left:50%;" class="fa fa-circle" aria-hidden="true"></i>'; 
                }

if($prod['stock'] == "ESGOTADO")
{
   echo '<span id="stock'.$prod['id'].'">'.$prod['stock'].'</span>';
   echo '<i style="color:#EA4335;position:relative;left:50%;" class="fa fa-circle" aria-hidden="true"></i>'; 
}
if($prod['stock'] == "APENAS POR ENCOMENDA")
{
    echo '<span id="stock'.$prod['id'].'">'.$prod['stock'].'</span>';
    echo '<i style="color:#FBBC05;position:relative;left:50%;" class="fa fa-circle" aria-hidden="true"></i>'; 
}
?>
</div>

    <div class="info-title-two" style="margin-left:5%;">
        PREÇO
    </div>

    <div class="info-info-two">
        <? echo floatval($prod['preco']); ?>€
    </div>


    <br><br>

    <div class="info-title-two">
    MARCA
    </div>

    <div class="info-info-two">
     <?
        echo $prod['marca'];
     ?>
    </div>

    <div class="info-title-two" style="margin-left:5%;">
        DIMENSÕES
    </div>

    <div class="info-info-two">
        <? echo $prod['dimensoes']; ?>
    </div>

    </div>

Jquery

    function addcart(x)
    {
        //FECHAR MODAL
        $("#modal-overlay"+x).click(function(){
            $("#modal-overlay"+x).hide();
            $("#modal-in"+x).hide();
        });

        var stock = $("#stock"+x).text();

        //VERIFICAR SE EXISTE STOCK
        if(stock == "EM STOCK" || stock == "APENAS POR ENCOMENDA")
        {
            $("#modal-in"+x).show();
            $("#modal-overlay"+x).show();

            //ADICIONAR NO CARRINHO
            $("#quant-enter"+x).off().on('click',function() {

                var prod = x;
                var quant = 0;
                var quant = $("#quant-val"+x).val();

                $.post('adicionar_item.php',
                {
                    prod:prod,
                    quant:quant
                },
                function(data)
                {
                    if(data == "404")
                    {
                        location.href = 'login.php';
                    }
                    else
                    {
                        $("#modal-in"+x).hide();
                        $("#modal-overlay"+x).hide();
                        $(".hue").text(data);
                        swal({   
                            title: "Adicionado!",   
                            text: "O produto foi adicionado ao seu carrinho", 
                            timer: 2000,  
                            imageUrl: "imagens/shop.png" 
                        });
                    }
                });
            });

        }
        else
        {
            swal("Erro!","O item escolhido não se encontra em stock","error");
        }
    }

P.S. To create unique id’s I create a static and add the primary key of the database table, then pass by the function.