upload_skip_duplicates.php

Alternative 1
----------------

/**
 * Prevent duplicate image uploads based on file name.
 */
function prevent_duplicate_image_upload( $file ) {
    // Only act on image uploads
    if ( strpos( $file['type'], 'image/' ) !== 0 ) {
        return $file;
    }

    $filename = basename( $file['name'] );

    // Strip the extension — this is what WordPress uses as post_title
    $title = pathinfo( $filename, PATHINFO_FILENAME );
    
    // Look for existing attachment with the same title
    $existing = get_page_by_title( $title, OBJECT, 'attachment' );
 

    if ( $existing ) {
        // Stop upload — show error message in the uploader
        $file['error'] = sprintf(
            'Upload blocked: The file "%s" already exists in the media library (Attachment ID: %d).',
            esc_html( $filename ),
            $existing->ID
        );
    }

    return $file;
}
add_filter( 'wp_handle_upload_prefilter', 'prevent_duplicate_image_upload' );


Alternative 2 by hsash
--------------------------

function prevent_duplicate_image_upload_by_hash( $file ) {
    if ( strpos( $file['type'], 'image/' ) !== 0 ) {
        return $file;
    }

    $new_file_hash = md5_file( $file['tmp_name'] );

    // Query attachments and compare hashes
    $attachments = get_posts( array(
        'post_type'      => 'attachment',
        'posts_per_page' => -1,
        'post_mime_type' => 'image',
        'fields'         => 'ids'
    ) );

    foreach ( $attachments as $attach_id ) {
        $existing_path = get_attached_file( $attach_id );
        if ( $existing_path && file_exists( $existing_path ) ) {
            if ( md5_file( $existing_path ) === $new_file_hash ) {
                $file['error'] = sprintf(
                    'Upload blocked: An identical image already exists (Attachment ID: %d).',
                    $attach_id
                );
                break;
            }
        }
    }

    return $file;
}
add_filter( 'wp_handle_upload_prefilter', 'prevent_duplicate_image_upload_by_hash' );


Alternative 3, probably not a good idea, by custom upload page
-------------------------



// Upload page for multiple image upload form
// Skipping images that exists already
// not sure what to restrict on, maybe:
// To restrict access (e.g., only logged-in users):
//if (!is_user_logged_in()) {
//    wp_die('You must be logged in to upload images.');
//}

// 1. Register a custom rewrite endpoint for the upload page
function custom_upload_page_rewrite() {
    add_rewrite_rule('^upload-images/?$', 'index.php?custom_upload_page=1', 'top');
}
add_action('init', 'custom_upload_page_rewrite');

// 2. Add query var for our endpoint
function custom_upload_page_query_vars($vars) {
    $vars[] = 'custom_upload_page';
    return $vars;
}
add_filter('query_vars', 'custom_upload_page_query_vars');

// 3. Display the upload form when visiting /upload-images/
function custom_upload_page_template() {
    if (intval(get_query_var('custom_upload_page')) === 1) {

        // Handle upload submission
        if (isset($_POST['custom_image_upload_nonce']) && wp_verify_nonce($_POST['custom_image_upload_nonce'], 'custom_image_upload')) {
            if (!function_exists('wp_handle_upload')) {
                require_once(ABSPATH . 'wp-admin/includes/file.php');
            }

            $upload_overrides = array('test_form' => false);
            $files = $_FILES['custom_images'];

            echo '<div style="padding:20px;font-family:sans-serif;">';

            foreach ($files['name'] as $key => $value) {
                if ($files['name'][$key]) {
                    $file = array(
                        'name'     => $files['name'][$key],
                        'type'     => $files['type'][$key],
                        'tmp_name' => $files['tmp_name'][$key],
                        'error'    => $files['error'][$key],
                        'size'     => $files['size'][$key]
                    );

                    $file_name = basename($file['name']);
                    $existing = get_page_by_title($file_name, OBJECT, 'attachment');

                    if ($existing) {
                        echo '<p style="color:red;">Skipped: <strong>' . esc_html($file_name) . '</strong> already exists (ID: ' . $existing->ID . ')</p>';
                        continue;
                    }

                    $movefile = wp_handle_upload($file, $upload_overrides);

                    if ($movefile && !isset($movefile['error'])) {
                        // Insert attachment
                        $attachment = array(
                            'guid'           => $movefile['url'],
                            'post_mime_type' => $movefile['type'],
                            'post_title'     => $file_name,
                            'post_content'   => '',
                            'post_status'    => 'inherit'
                        );
                        $attach_id = wp_insert_attachment($attachment, $movefile['file']);

                        require_once(ABSPATH . 'wp-admin/includes/image.php');
                        $attach_data = wp_generate_attachment_metadata($attach_id, $movefile['file']);
                        wp_update_attachment_metadata($attach_id, $attach_data);

                        echo '<p style="color:green;">Uploaded: <strong>' . esc_html($file_name) . '</strong> (ID: ' . $attach_id . ')</p>';
                    } else {
                        echo '<p style="color:red;">Error uploading <strong>' . esc_html($file_name) . '</strong>: ' . esc_html($movefile['error']) . '</p>';
                    }
                }
            }

            echo '</div>';
        }

        // Output the HTML form.
        ?>
        <div style="max-width:600px;margin:50px auto;padding:20px;border:1px solid #ccc;border-radius:10px;font-family:sans-serif;">
            <h2>Upload Images</h2>
            
            <p>Avoid duplicate files upload</p>
        
            <form method="post" enctype="multipart/form-data">
                <?php wp_nonce_field('custom_image_upload', 'custom_image_upload_nonce'); ?>
                <p><label>Select Images:</label><br>
                <input type="file" name="custom_images[]" accept="image/*" multiple required></p>
                <input type="submit" value="Upload Images">
            </form>
        </div>
        <?php

        exit; // Stop WordPress from loading a normal page template
    }
}
add_action('template_redirect', 'custom_upload_page_template');