To Display An Alert Message While Inserting A Duplicate Record

I am duplicating a record from my asp.net page in to the database. When i click on save I am getting the following error message

Violation of PRIMARY KEY constraint 'PK_clientinfo'. Cannot insert duplicate key in object 'clientinfo'. The statement has been terminated.

The above message i am getting since i have tried to duplicate the clientname field of my table which is set as the primary key.

What i want is instead of this message in the browser i need to display an alert saying "the clientname entered already exists" by checking the value from the database.

Here is my code. pls modify it to achieve the said problem

if(Page.IsValid==true)
    {      
     conn.Open();
     SqlCommand cmd = new SqlCommand("insert into clientinfo (client_name, address) values ('"+txtclientname.Text+"', '"+txtaddress.Text+"')", conn);
     cmd.ExecuteNonQuery();
     conn.Close();
     BindData();
     txtclear(); 
     System.Web.HttpContext.Current.Response.Write("<script>alert('New Record Added!');</script>");
     Response.Redirect("Clientinfo.aspx");
    }