private static ConcurrentDictionary<string, string> s_urlToContents=new ConcurrentDictionary<string, string>();

public static async Task<string> GetContentsAsync(string url)
{
    if (!s_urlToContents.TryGetValue(url, out string contents))
    {
        var response = await new HttpClient().GetAsync(url);
        contents = await response.EnsureSuccessStatusCode().Content.ReadAsStringAsync();
        s_urlToContents.TryAdd(url, contents);
    }
    return contents;
}

static void Main(string[] args)
{
    Task<string> task=GetContentsAsync(@"https://ya.ru");
    task.Wait();
    string respons = task.Result;
    Console.WriteLine(respons);

    Console.ReadLine();
}

private static ConcurrentDictionary<string, Task<string>> s_urlToContents=new ConcurrentDictionary<string, Task<string>>();

public static Task<string> GetContents(string url)
{
    if (!s_urlToContents.TryGetValue(url, out Task<string> contents))
    {
        contents = GetContentsAsync(url);
        contents.ContinueWith(
            delegate { s_urlToContents.TryAdd(url, contents); },
            CancellationToken.None,
            TaskContinuationOptions.OnlyOnRanToCompletion | TaskContinuationOptions.ExecuteSynchronously,
            TaskScheduler.Default);
    }
    return contents;
}

private static async Task<string> GetContentsAsync(string url)
{
    var response = await new HttpClient().GetAsync(url);
    return await response.EnsureSuccessStatusCode().Content.ReadAsStringAsync();
}