Multiple postmeta with same name for one post in wp_postmeta table

Question

I have a hidden post meta called _payment for my posts. When i was looking into the tables of the wordpress, I noticed that there are tens of this post meta with the same name for the same post! Here is a picture of the database:

I don't think I write any wrong code to cause this, I just added and updated the field, but it should update the same field with the same meta_id, not that create a new one with a different meta_id for the same post.

But still I don't know, maybe it's a normal wordpress thing? Or I'm wrong ...

Here are the codes I'm using for modifying this field:

function payment_meta($post_ID)
{
    $user_id = get_post_field( 'post_author', $post_ID );
    $user_role = 'editor'; 
    $post_type = 'portfolio'; 
    if( get_post_type( $post_ID ) == $post_type ) {
        if ( get_post_meta($post_ID, '_payment', true) != '1' && !user_can( $user_id, $user_role )) {
            add_post_meta($post_ID, '_payment', '0');
        }
        else if ( get_post_meta($post_ID, '_payment', true) == '' && user_can( $user_id, $user_role )) {
            add_post_meta($post_ID, '_payment', '1');
        }
    }
}

add_action('wp_insert_post', 'payment_meta', 11, 1);

And this code, this runs a cron job when the a post expires :

function unpublish_post($post_id)
{
    wp_update_post(array(
        'ID' => $post_id,
        'post_status' => 'pending'
    ));

    update_post_meta($post_id, '_payment', '0');
}

add_action('expiration_date', 'unpublish_post', 11, 1);

Answer 1

wp_insert_post runs whenever a post is updated, not just when it's first saved. So your payment_meta() function is running every time a post is saved, and since you're using add_post_meta() in that function, a new meta entry is created every time the post is saved.

Instead of using add_post_meta() stick to just using update_post_meta() if a value isn't already set this will still create the meta, but if it already exists it will update the existing value, rather than adding a new one:

function payment_meta($post_ID)
{
    $user_id = get_post_field( 'post_author', $post_ID );
    $user_role = 'editor'; 
    $post_type = 'portfolio'; 
    if( get_post_type( $post_ID ) == $post_type ) {
        if ( get_post_meta($post_ID, '_payment', true) != '1' && !user_can( $user_id, $user_role )) {
            update_post_meta($post_ID, '_payment', '0');
        }
        else if ( get_post_meta($post_ID, '_payment', true) == '' && user_can( $user_id, $user_role )) {
            update_post_meta($post_ID, '_payment', '1');
        }
    }
}

add_action('wp_insert_post', 'payment_meta', 11, 1);