Doubts question and answer systems with $ _GET method

I have a system that shows a question and the person responds, but I am with a failure, following:  When the staff responds step I use the $ _GET method to pick up the response from it and insert the information into the bank, plus the failure and that if the person refreshes the page or something the information is inserted 2x into the bank, as it could do to do only an inclusion in the bank?

Form:

<form class="form-horizontal" action="comportamento.php?nro_pergunta=<?php  echo $nro_pergunta; ?>" method="GET">
  <fieldset>
      <legend><b>Pergunta <?php echo"$ordem";?> de <?php echo"$totalpergunta";?></b></legend>
    <?php
     $timestamp            = @mktime(date("H")-4, date("i"),date("s"),date("m"),date("d"),date("Y"),0);
     $data_cad              = gmdate("Y-m-d", $timestamp);
     echo"<input type='hidden' name='loja'         id='loja'         value='$loja'>";
     echo"<input type='hidden' name='id_usuario'   id='id_usuario'   value='$id_usuario'>";
     echo"<input type='hidden' name='questionario' id='questionario' value='$questionario'>";
     echo"<input type='hidden' name='area'         id='area'         value='$area'>";
     echo"<input type='hidden' name='nro_pergunta' id='nro_pergunta' value='$nro_pergunta'>";     
     echo"<input type='hidden' name='data_cad'     id='data_cad'     value='$data_cad'>";
     echo"<input type='hidden' name='pergunta'     id='pergunta'     value='$idpergunta'>";
    ?>
       <?php
        $sql_button = mysql_query("SELECT *  FROM respostas WHERE 
           resp_loja='$loja'
       AND resp_usuario='$id_usuario'
       AND resp_area ='$area'
       AND resp_pergunta_id ='$idpergunta'
       AND resp_subgrupo='100'
       AND resp_questionario='$questionario'");

     if(mysql_num_rows ($sql_button) == 0 )
      {
     echo"<div class='form-group'>";
      echo"<center><h4><label> $descpergunta</label></h4></center>";
      echo"<div class='col-lg-10'>";
        echo"<div class='radio'><label><input type='radio' name='resp' id='optionsRadios1' value='5'>OTIMO        </label></div>";
        echo"<div class='radio'><label><input type='radio' name='resp' id='optionsRadios2' value='3'>BOM      </label></div>";
        echo"<div class='radio'><label><input type='radio' name='resp' id='optionsRadios3' value='2'>REGULAR         </label></div>";
        echo"<div class='radio'><label><input type='radio' name='resp' id='optionsRadios3' value='1'>RUIM         </label></div>";
        echo"<div class='radio'><label><input type='radio' name='resp' id='optionsRadios4' value='0'>NAO APLICADO </label></div>";
        echo"</div>";
      echo"</div>";
     echo"<div class='form-group'>";
      echo"<div class='col-lg-10 col-lg-offset-2'>";
                echo"<button id='button' type='submit' name='submit' class='btn btn-danger'><span class='glyphicon glyphicon-ok'></span> Responder</button><br />";

       echo"</div>";
    echo"</div>";
    } else  {
    echo"<div class='form-group'>";
      echo"<div class='col-lg-10 col-lg-offset-2'>";
            echo"<button id='button' type='next' name='next' class='btn btn-danger'><span class='glyphicon glyphicon-circle-arrow-right'></span> Próxima</button><br />";
            echo"<br>";
            echo"<div class='alert alert-dismissible alert-danger'>";
            echo"<button type='button' class='close' data-dismiss='alert'></button>";
            echo"<strong>ATENÇÃO! </strong>Você já respondeu essa pergunta.";
            echo"</div>";  
       echo"</div>";
    echo"</div>";
    }
   ?> 


  </fieldset>
 </form>

Insert:

<?php
 include("conexao.php");
if(isset($_GET['submit'])){
    $loja         = $_GET ["loja"];
    $id_usuario   = $_GET ["id_usuario"]; 
    $questionario = $_GET ["questionario"];
    $area         = $_GET ["area"];
    $idpergunta   = $_GET ["pergunta"];   
    $resp         = $_GET ["resp"]; 
    $datacad      = $_GET ["data_cad"]; 

  $sql_insert = mysql_query("INSERT INTO respostas 
  (resp_id,resp_loja,resp_usuario,resp_questionario,resp_area,resp_subgrupo,resp_pergunta_id,resp_resposta,resp_data,resp_status)
  VALUES 
  ('','$loja','$id_usuario','$questionario','$area','100','$idpergunta','$resp','$datacad','1')") or die (mysql_error());
    mysql_query($sql_insert,$conexao); } 

if(isset($_GET['proxima'])){
$pergunta = (int)$_GET['proxima'];
header('location: comportamento.php?nro_pergunta='.$pergunta);  
}

NOTE: I've just added the part of the code for the question, but if you need any more information please return.

And to conclude, I do not know if you can put 2 questions together, but if you can answer, it's as follows: In the inclusion of the database is inserting only the date, and not the date as it is in the field, would the inclusion form be wrong?

No more a strong hug and even more.