how to restrict duplicates when importing the excel data to database? (visual c)

I want to restrict duplicates while uploading the excel data and to inform the user that the particular cell in excel contains duplicate value which is already exist in Database. I have researched online regarding this problem and know one of the way is to insert my excel data to a temp table, then compare the data with the destination table and return the duplicates back to the user so that he can fix them and re-upload the file. But i do not know how do that without a code example.. Or is there any other way to do it? I would really appreciate your help!

(Duplicates should be checked only in Student_ID(pk), the student can have the same Student_name, Student_class .If duplicate record is found, Message should display as:"User cannot upload the data to with duplicate values. Kindly remove the duplicate fields and re-upload."

I've already successfully done the importing part using SqlBulkCopy

**This is my importing excel file to database code**

protected void Button1_Click(object sender, EventArgs e)
    {
        if (FileUpload1.HasFile)
        {
            string path = string.Concat((Server.MapPath("~/temp/" + FileUpload1.FileName)));
            FileUpload1.PostedFile.SaveAs(path);
            OleDbConnection OleDbcon = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + path + ";Extended Properties=Excel 12.0;");
            OleDbCommand cmd = new OleDbCommand("Select * from [Sheet1$]", OleDbcon);
            OleDbDataAdapter ObjAdapter1 = new OleDbDataAdapter(cmd);

            OleDbcon.Open();
            DbDataReader dr = cmd.ExecuteReader();
            string con_str = @"Data Source=xx;Initial Catalog=BSS;Integrated Security=True";

            SqlBulkCopy bulkinsert = new SqlBulkCopy(con_str);
            bulkinsert.DestinationTableName = "Student";
            bulkinsert.WriteToServer(dr);
            OleDbcon.Close();
            Array.ForEach(Directory.GetFiles((Server.MapPath("~/temp/"))), File.Delete);
            Label1.ForeColor = Color.Green;
            Label1.Text = "Successfully inserted";
            FetchData();

        }

        else
        {
            Label1.ForeColor = Color.Red;
            Label1.Text = "Please select the file";


        }
    }

    public void FetchData()
    {
        SqlConnection con = new SqlConnection("xx";Initial Catalog=BSS;Integrated Security=True");
        SqlCommand cmd = new SqlCommand("select * from Student", con);
        con.Open();
        SqlDataReader rdr = cmd.ExecuteReader();
        GridView1.DataSource = rdr;
        GridView1.DataBind();


    }
    }

And this is my aspx design code:

<%@ Page Title="" Debug="true" Language="C#" MasterPageFile="~/MasterPage2.master" AutoEventWireup="true" CodeFile="TeacherImport.aspx.cs" Inherits="StudentImport" %><asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server"></asp:Content><asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server"><p><br /></p><asp:FileUpload ID="FileUpload1" runat="server" /><asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="import" /><br /><br /><asp:Label ID="Label1" runat="server" Text="Label"></asp:Label><br /><br /><asp:GridView ID="GridView1" runat="server" Width="1055px"></asp:GridView><br /><br /><br /></asp:Content>

Thank you!