EDITED
I wrote an if in the php where if $consulta fetches a row then it throws an echo, but if it does not load a row with the insert
the problem is the if condition dont do nothing, every time I do the query insert the row, even if there is already an equal row. whats wrong?
function vincular(){
  $.ajax({
          url:   'api/vinculados.php',
          type:  'POST',
          data: {
            juridica: $("#sel_jur").val(),
            fisica: $("#mod_id_perfil").val(),
            usuario:  $("#mod_usuario_perfil").val()
          },
          success:  function (data) {
            if(data.status == "ok") {
              console.log(data);
            }
          }
  });
}<?php
include 'conexion.php';
$consulta=mysqli_query($conexion, "SELECT * from perfiles_vinculados WHERE perfil_juridica = '$_POST[juridica]'");
if (mysqli_num_rows($consulta) > 0)
{
echo "ya existe";
} else {
  mysqli_query($conexion, "INSERT INTO perfiles_vinculados (perfil_juridica, perfil_fisica, usuario)
   VALUES ('$_POST[juridica]', '$_POST[fisica]', '$_POST[usuario]')");
}
 ?> 
    