Need to remove duplicate values from C# program

Question

I need some help with a C# program that i am creating. So in this scenario i am inputting duplicate values into the program. For Example, a,b,b,c,c.
The exercise is that

Answer 1

Using a hashtable (Generic Dictionary) is an efficient way to determine if an entered character has already been encountered.

Also, the Char.IsLetter method in the .NET framework is a great way to check for bad data.

static void Main(string[] args) {
    Dictionary charsEntered = new Dictionary();

    Console.WriteLine("Please enter 5 characters, each on a separate line.");
    while (charsEntered.Count() < 5) {
        Console.WriteLine("Enter a character:");
        char[] resultChars = Console.ReadLine().ToCharArray();

        if(resultChars.Length != 1 || !Char.IsLetter(resultChars[0])) {
            Console.WriteLine("Bad Entry. Try again.");
        } else {
            char charEntered = resultChars[0];
            if (charsEntered.ContainsKey(charEntered))
                Console.WriteLine("Character already encountered. Try again.");
            else
                charsEntered[charEntered] = true;
        }
    }

    Console.WriteLine("The following inputs were entered:");
    Console.WriteLine(String.Join(", ", charsEntered.Keys));
    Console.ReadLine();
}