add_post_meta unique value for all custom posts

you can use the save_post hook to validate the uniqueness of email and phone fields before saving the post.

Replace seller with your actual post type slug and add below code in functions.php file

// Add custom meta box for Seller post type
function add_seller_meta_box() {
    add_meta_box(
        'seller_meta_box',
        'Seller Details',
        'render_seller_meta_box',
        'seller',
        'normal',
        'default'
    );
}
add_action('add_meta_boxes', 'add_seller_meta_box');

// Render the custom meta box fields
function render_seller_meta_box($post) {
    // Add your custom fields HTML inputs here
    echo '<label for="seller_email">Email:</label>';
    echo '<input type="text" id="seller_email" name="seller_email" value="' . get_post_meta($post->ID, 'seller_email', true) . '" /><br>';

    echo '<label for="seller_phone">Phone:</label>';
    echo '<input type="text" id="seller_phone" name="seller_phone" value="' . get_post_meta($post->ID, 'seller_phone', true) . '" /><br>';
}

// Save custom meta box data
function save_seller_meta_box_data($post_id) {
    // Check if it's not an autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return;

    // Check if the current user has permission to edit the post
    if (!current_user_can('edit_post', $post_id)) return;

    // Check if it's the correct post type
    if ('seller' !== get_post_type($post_id)) return;

    // Validate uniqueness of email and phone fields
    $email = isset($_POST['seller_email']) ? sanitize_email($_POST['seller_email']) : '';
    $phone = isset($_POST['seller_phone']) ? sanitize_text_field($_POST['seller_phone']) : '';

    // Check if email or phone already exists
    $existing_sellers = get_posts(array(
        'post_type' => 'seller',
        'post_status' => 'publish',
        'meta_query' => array(
            'relation' => 'OR',
            array(
                'key' => 'seller_email',
                'value' => $email,
                'compare' => '='
            ),
            array(
                'key' => 'seller_phone',
                'value' => $phone,
                'compare' => '='
            )
        )
    ));

    if (!empty($existing_sellers)) {
        wp_die('A seller with this email or phone already exists.', 'Duplicate Seller', array('response' => 400));
    }

    // Save meta data
    update_post_meta($post_id, 'seller_email', $email);
    update_post_meta($post_id, 'seller_phone', $phone);
}
add_action('save_post', 'save_seller_meta_box_data');