.net - Catching An Exception In C# / "Already There Is A Same Data" Or Any Words?

I have a simple button ADD

And ADD_Click code is:

protected void Add_Click(object sender, EventArgs e)
{
string strConnectionString = ConfigurationManager.ConnectionStrings["SqlServerCstr"].ConnectionString;
SqlConnection myConnection = new SqlConnection(strConnectionString);
myConnection.Open();
string hesap = Label1.Text;
string musteriadi = DropDownList1.SelectedItem.Value;
string avukat = DropDownList2.SelectedItem.Value;
SqlCommand cmd = new SqlCommand("INSERT INTO AVUKAT VALUES (@MUSTERI, @AVUKAT, @HESAP)", myConnection);
cmd.Parameters.AddWithValue("@HESAP", hesap);
cmd.Parameters.AddWithValue("@MUSTERI", musteriadi);
cmd.Parameters.AddWithValue("@AVUKAT", avukat);
cmd.Connection = myConnection;
SqlDataReader dr = cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection);
Response.Redirect(Request.Url.ToString());
myConnection.Close();
}

What i want is, if anybody adding a data with same HESAP (primary key) i want a alert() function with this words. "Already there is a same data" or any words. Doesn't matter.

How can i do that?

NOTE: I want just the same HESAP value getting and catching an error.