stop duplicate record in database

Question

I need to stop duplicate records in my sql table and if  data exists how can i show message says already exists this is my code below :

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Windows.Forms;

using System.Data;

using System.Data.SqlClient;

 

namespace cap

{

public partial class WebForm1 : System.Web.UI.Page

{

SqlConnection con;

protected void Page_Load(object sender, EventArgs e)

{

}

protected void Button1_Click1(object sender, EventArgs e)

{

SqlConnection con = new SqlConnection(@"Data Source=DESKTOP-6I0G8AT;Initial Catalog=cap;Integrated Security=True");

con.Open();

string store = "INSERT INTO Customer (Cust_Name,Cust_Username,Cust_Email,Cust_Contact,Cust_Password ) VALUES (@Cust_Name,@Cust_Username,@Cust_Email,@Cust_Contact,@Cust_Password)";

SqlCommand cmd = new SqlCommand(store,con);

cmd.Parameters.AddWithValue("@Cust_NAME", txtfirstname.Text + " " + txtlastname.Text);

cmd.Parameters.AddWithValue("@Cust_Username", txtusername.Text);

cmd.Parameters.AddWithValue("@Cust_Email", txtemail.Text);

cmd.Parameters.AddWithValue("@Cust_Contact", txtcontact.Text);

cmd.Parameters.AddWithValue("@Cust_Password", txtpassword.Text);

cmd.ExecuteNonQuery();

con.Close();

}

}

}