C# - How to Make the ComboBox ' Values Distinct (Unique) without using any database?

Question

i have the following code with me- it populates all the values and i want the distinct one. please help me to resolve it.
THANKS IN ADVANCE....

foreach (string file in System.IO.Directory.GetFiles(a))

            {

string asd = Path.GetFileNameWithoutExtension(file);

                string[] str = asd.Split('_');

string sam1 = str[0];

                string sam2 = str[1];

                combo1.Items.Add(sam1);

                combo2.Items.Add(sam2);

            }