Prevent upload in WordPress media library if there is already a file with the same name.

wp-upload-existing-filename-filter.php

add_filter( 'wp_handle_upload_prefilter', function ( $file ) {
    $uploads       = wp_upload_dir();
    $use_yearmonth = get_option( 'uploads_use_yearmonth_folders' );
    if ( boolval( $use_yearmonth ) ) {
        // if upload to year month based folders is enabled check current target
        $year   = date( 'Y' );
        $month  = date( 'm' );
        $target = $uploads['path'] . DIRECTORY_SEPARATOR . $year . DIRECTORY_SEPARATOR . $month . DIRECTORY_SEPARATOR . $file['name'];
    } else {
        // uploads dir
        $target = $uploads['path'] . DIRECTORY_SEPARATOR . $file['name'];
    }
    if ( file_exists( $target ) ) {
        $file['error'] = 'File with the same name already exists.';
    }

    return $file;
} );