violation of primary key constraint pk_T_registration, cannot insert duplicate key in the object dbo.T_registration error

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ASP.NET

SQL-Server

LINQ

i created one registyration table with pateintid as primary key with with autoincrement 1.
i am using linq to submit my deatails like name,gender....the problem was , only one record is inserting in the table starts pateintid with 0 when i try to insert second record above error is coming..........plz give some idea

here is my code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

public partial class PatientRegistration : System.Web.UI.Page
{

ADO objado = new ADO();
DataClassesDataContext objlinq = new DataClassesDataContext();
T_patient_registration objreg = new T_patient_registration();





protected void Page_Load(object sender, EventArgs e)
{

}

protected void Button1_Click(object sender, EventArgs e)
{
objreg.name = txtpatientname.Text;
objreg.father_mothername = txtguardianname.Text;
objreg.gender = drpgender.SelectedItem.ToString();
objreg.dateofbirth = txtdob.Text;
objreg.disease = txtdisease.Text;
objreg.age = Convert.ToInt32(txtage.Text);
objreg.emailid = txtemail.Text;
objreg.currentaddress = txtcaddress.Text;
objreg.city = drpcity.SelectedItem.ToString();
objreg.contactnumber = Convert.ToInt64(txtcontact.Text);
objreg.permanentaddress = txtpaddress.Text;
objreg.specialist = drpspecialist.SelectedItem.ToString();
objreg.occupation = drpoccupation.SelectedItem.ToString();
objlinq.T_patient_registrations.InsertOnSubmit(objreg);
objlinq.SubmitChanges();
Label1.Text = "Data Inserted Successfully";

}

}