How To Prevent A Second Entry Being Inserted With Primary Key Value? (C#)

Ok, this is a really stupid question, but I can't seem to find an answer I understand. In my SQL database I have a a table called MasterSkillList, to which the user can write by using a little web form with a text box and a drop down list. The table has 2 fields, Skill and Attribute. Skill is the primary key, as no skill can appear twice. What I want to do is prevent just that, I don't want people to enter the same skill more than once. So how do I tell the user that the entry allready exists in the database?

 My C#  Code is as follows:1 protected void btnSubmit_Click(object sender, EventArgs e)
2 {
3 srcAddSkill.InsertParameters["Skill"].DefaultValue = txtSkillName.Text;
4 srcAddSkill.InsertParameters["Attribute"].DefaultValue = ddlAbility.SelectedValue;
5 try
6 {
7 srcAddSkill.Insert();
8 lblErrorMessage.Text = "The skill '" + txtSkillName.Text + "' has been added. It is based on a character's " + ddlAbility.SelectedItem + " score.";
9 lblErrorMessage.Visible = true;
10 txtSkillName.Text = "";
11 }
12 catch (Exception ex)
13 {
14 lblErrorMessage.Text = "An exception has occurred. " + ex.Message;
15 lblErrorMessage.Visible = true;
16 }