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

Keep it simple, and although a HashSet is nice semantically, it's not needed for 5 elements (it's actually slower than a List in that case). Worse, it requires a parallel structure to track the characters (assuming you care about order).

Clearly none of these considerations matter for such a small example but it's good to learn them up front and don't always jump to big-O notation when actually measured performance and memory consumption should be your guide for most practical applications.

Instead you can simply do:-

List chars = new List(5);
while (chars.Count < 5)
{
   char c = Console.ReadKey().KeyChar;
   if (!char.IsLetter(c)) continue;
   if (chars.Contains(char)) continue;
   chars.Add(char);
}

Plus whatever error messages you want to add.