Pasted

public static List<string> NormalizeUrls(List<string> urls, string rootUrl)
{
    Uri uri;
    if (!Uri.TryCreate(rootUrl, UriKind.Absolute, out uri)) return new List<string>();
    return urls.Select(url => !Uri.IsWellFormedUriString(
        url, UriKind.Absolute) ? 
        string.Concat(uri.AbsoluteUri, url) : url).Distinct().ToList();
}