Program.cs

class Program
{
    static void Main(string[] args)
    {
        // John responded to Michael 20 times
        // John responded to Dave 10 times
        // Michael responded to John 5 times
        // Michael responded to Dave 8 times
        // Dave responded to John 19 times
        // Dave responded to Michael 3 times

        var users = new Dictionary<string, User>
        {
            {"John", new User("John")},
            {"Michael", new User("Michael")},
            {"Dave", new User("Dave")}
        };

        users["John"].Replies["Michael"] = 20;
        users["John"].Replies["Dave"] = 10;
        users["Michael"].Replies["John"] = 5;
        users["Michael"].Replies["Dave"] = 8;
        users["Dave"].Replies["John"] = 19;
        users["Dave"].Replies["Michael"] = 3;

        var communicationTally = new Dictionary<(string, string), int>();

        foreach (var a in users)
        {
            foreach (var b in a.Value.Replies)
            {
                var foundPair = false;

                foreach (var cTallyKvp in communicationTally)
                {
                    // Check if there is an already existing item in communicationTally that
                    // represents this pair of users.
                    if ((cTallyKvp.Key.Item1 == a.Key && cTallyKvp.Key.Item2 == b.Key) ||
                        (cTallyKvp.Key.Item1 == b.Key && cTallyKvp.Key.Item2 == a.Key))
                    {
                        foundPair = true;
                        communicationTally[cTallyKvp.Key] += b.Value;

                        break;
                    }
                }

                if (!foundPair)
                {
                    communicationTally.Add((a.Key, b.Key), b.Value);
                }
            }
        }

        foreach (var kvp in communicationTally)
        {
            Console.WriteLine($"{kvp.Key.Item1} and {kvp.Key.Item2} communicated {kvp.Value} times.");
        }

        Console.WriteLine("Press the any key to continue...");
        Console.ReadKey();
    }

    private class User
    {
        public string UserName { get; }
        public Dictionary<string, int> Replies { get; }

        public User(string userName)
        {
            UserName = userName;
            Replies = new Dictionary<string, int>();
        }
    }
}