Whenever I Click Rigister Button Two Time Data Is Inserted. How To Avoid It?

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C#

protected void btnregister_Click(object sender, EventArgs e)
   {
       con.Open();
       {
           cmd.CommandText = "usp_adduser";
           cmd.CommandType = CommandType.StoredProcedure;
           cmd.Parameters.AddWithValue("@user_name", txtusername.Text);
           cmd.Parameters.AddWithValue("@password", txtpassword.Text);
           cmd.Parameters.AddWithValue("@firstname", txtfname.Text);
           cmd.Parameters.AddWithValue("@lastname", txtlname.Text);
           cmd.Connection = con;
           cmd.ExecuteNonQuery();
           int check = 0;
           check = cmd.ExecuteNonQuery();
           if (check == 1)
           {
               con.Close();
           }
           else
           {
               Response.Write("Error");
           }
       }
       con.Close();
   }

this is above code.


ALTER PROCEDURE dbo.usp_adduser
--@user_id int = 1,
@user_name varchar(50),
@password varchar(50),
@firstname varchar(50),
@lastname varchar(50)


AS
begin

insert into registeruser (user_name,password,firstname,lastname) values (@user_name,@password,@firstname,@lastname)

end

and the above is the stored procedure..