Multiple meta values for same meta_key adding on “Preview Changes” hit but not on saving or updating post

I’m taking multiple location values with each post, so it’s an array (always). My post meta form field is like so:

<select name="<?php echo $project_prefix; ?>offer_locations[]" id="<?php echo $project_prefix; ?>offer_locations" required multiple="multiple">
    ...
    ...
    ...
</select>

If the post already exists without any value for locations, the existing value will come null, but as the field is mandatory, the new location value [on saving] will surely be an array, and we should add the postmeta value.

The conditionals I made with array_diff() is working fine. I made a custom scenario and checked the code, and even in WordPress, I’ve checked the code (custom code block), as you can see couple of var_dump(), echo and exit() inline. But…

<?php
function save_post_specifics_meta( $post_id ) {
    global $project_prefix;

    // verify nonce
    if (!isset($_POST['post_specifics_nonce']) || !wp_verify_nonce($_POST['post_specifics_nonce'], basename(__FILE__)))
       return $post_id;

    // check autosave
    if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE )
       return $post_id;

    // check permissions
    if ( 'post' == $_POST['post_type'] && !current_user_can('edit_post', $post_id) )
    return $post_id;

    //Location meta data
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );
    $new_locations      = $_POST["{$project_prefix}post_locations"]; //always an array

    var_dump($existing_locations);  //for debugging only
    var_dump($new_locations);       //for debugging only

    if( is_array( $existing_locations ) ) {
        $new_diff       = array_diff( $new_locations, $existing_locations );
        $existing_diff  = array_diff( $existing_locations, $new_locations );
    } else {
        $new_diff       = $new_locations;
        $existing_diff  = $existing_locations;
    }
    var_dump($new_diff);            //for debugging only
    var_dump($existing_diff);       //for debugging only

    if( $new_diff ) {
        foreach( $new_diff as $new_location ) {
            add_post_meta( $post_id, "{$project_prefix}post_locations", $new_location );
            echo "New Multiple: ", $new_location, '<br>'; //for debugging only
        }
    }

    if( $existing_locations && $existing_diff ) {
        if( is_array( $existing_diff ) ) {
            foreach ( $existing_diff as $existing_location ) {
                delete_post_meta( $post_id, "{$project_prefix}post_locations", $existing_location );
                echo "Delete Multiple: ", $existing_location, '<br>'; //for debugging only
            }
        } else {
            delete_post_meta( $post_id, "{$project_prefix}post_locations", $existing_diff );
            echo "Delete Single: ", $existing_diff, '<br>'; //for debugging only
        }
    }

    exit(); //for debugging only

}

add_action( 'save_post',        'save_post_specifics_meta', 10 );
add_action( 'new_to_publish',   'save_post_specifics_meta', 10 );

On saving or updating post, location meta field is doing as expected. But if the data entry operator hit the Preview Changes button, the add_post_meta() is triggering, even within the true conditional braces, and adding duplicate entries, I don’t know how!!! 😮

I then added true to the last parameter (line#40) to make the value unique, so that I can stop duplicate entries. But adding true stopped adding any new value to the meta_key even on saving.

How can I stop adding meta value on Preview Changes hit?

Actually I’d love to have an answer from @ialocin, because he suggested me the way. But as the code is a bit long for comment field I’m posting it here as an answer. But I’d love to accept an answer more appropriate to the situation:

After @ialocin’s suggestion, I got that, the $post_id wasn’t accurate for a Preview Changes hit, so the $existing_locations wasn’t getting the correct db info. Now I changed the following line:

$existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );

into the following conditional lines:

if( $_POST[ 'wp-preview' ] === 'dopreview' ) {
    //if it's a preview hit, grab the actual post's data
    $actual_post_id     = wp_get_post_parent_id( $post_id );
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $actual_post_id );
} else {
    //else, go with the actual post
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );
}

Everything is working as expected in localhost. Though sometimes, the Preview hit fails the preview_id and preview_nonce etc. and doesn’t show the preview, but when they’re set, the preview is, exactly what it should be.