C# sql how do i prevent multiple usernames being inserted

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C#

SQL

I have a form in my windows form application that is for admin users so that they can add,edit and delete users. The problem i have is that admin can insert a new user which has the same username as another user which results in both users having duplicate usernames when logging into the system.

how can i prevent admin from inserting an already taken username when creating new accounts in the database.

SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|Data.mdf;Integrated Security=True");
                con.Open();
                SqlDataAdapter sda = new SqlDataAdapter("INSERT INTO Login (FirstName,Role,Username,Password,Surname) VALUES ('" + txtfirstname2.Text + "','" + rolecombo.Text + "','" + txtusername2.Text + "','" + txtpassword2.Text + "','" + txtsurname.Text + "')", con);
                sda.SelectCommand.ExecuteNonQuery();
                con.Close();
                MessageBox.Show("SAVED SUCCESSFULLY !!!!!");

What I have tried:

I have looked across many articles which suggested adding an identity to usernames to make them unique, since i already have an ID column in the table which has an identity, i can not use the solution so therefore I need a simpler solution such as the system throwing a messagebox if username is duplicate but i'm not sure how to do that.