<?php
$service = '';
$query = "SELECT service FROM services GROUP BY service";
$result = mysqli_query($con, $query);
while($row = mysqli_fetch_array($result))
{
$service .= '<option
value="'.$row["service"].'">'.$row["service"].'</option>';
}
?>

<select name="service" id="service" class="form-control action">
<option value="">Выберите тип услуги</option>
<?php echo $service; ?>
</select>

<?php
$sql = "SELECT service FROM services GROUP BY service";
$result = mysqli_query($con, $query);
$service = array();
while($row = mysqli_fetch_array($result)){
    $service[] = $row;
}
echo json_encode($service, JSON_UNESCAPED_UNICODE);
?>

$(function(){
    function selectUpdate() {
        $.ajax({
            url: 'service.php',
            async:false,
            success: function(data){
                json = JSON.parse(data);
                for (i = 0; i < json.length; i++)
                    $('#service').append('<option value="'+json[i].service+'">'+json[i].service+'</option>');
            }
        });
    }
    selectUpdate();
    $('#service').on('change', function(){
        selectUpdate();
    });
});