How to prevent user from adding the duplicate records for same date in ASP.Net

Yogesh here is smiple sample for.

Html Code

<asp:GridView ID="gvStudents" runat="server" AutoGenerateColumns="false">
            <Columns>
                <asp:TemplateField HeaderText="Roll No">
                    <ItemTemplate>
                        <asp:Label ID="lblRollNo" runat="server" Text='<%#Eval("RollNo") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Student Name">
                    <ItemTemplate>
                        <asp:Label ID="lblStudentName" runat="server" Text='<%#Eval("StudentName") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Attendence">
                    <ItemTemplate>
                        <asp:CheckBox ID="chkAttendence" runat="server" />
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>
        <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
        <asp:Button ID="btnSave" runat="server" Text="Save" OnClick="SaveStudentAttendance" />

Code

  protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            this.txtDate.Text = DateTime.Now.ToShortDateString();
            this.PopulateStudentDetails();
        }

    }

    private void PopulateStudentDetails()
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conString"].ConnectionString);
        SqlDataAdapter da = new SqlDataAdapter("SELECT RollNo,StudentName FROM Table_Student", con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        gvStudents.DataSource = ds.Tables[0];
        gvStudents.DataBind();
    }

    protected void SaveStudentAttendance(object sender, EventArgs e)
    {
        bool IsAvailable = false;
        foreach (GridViewRow row in gvStudents.Rows)
        {
            if (row.RowType == DataControlRowType.DataRow)
            {
                CheckBox chkAttendance = row.FindControl("chkAttendence") as CheckBox;
                string attendanceStatus = chkAttendance.Checked ? "Present" : "Absent";
                int rollNo = int.Parse((row.FindControl("lblRollNo") as Label).Text.Trim());
                string studentName = ((row.FindControl("lblStudentName") as Label).Text.Trim());
                string sttendenceDate = (this.txtDate.Text.Trim());
                string constring = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
                using (SqlConnection con = new SqlConnection(constring))
                {
                    using (SqlCommand cmd = new SqlCommand())
                    {
                        cmd.CommandType = CommandType.StoredProcedure;
                        cmd.CommandText = "SaveAttendence";
                        cmd.Connection = con;
                        con.Open();
                        cmd.Parameters.AddWithValue("@StudentName", studentName);
                        cmd.Parameters.AddWithValue("@StudentRollNo", rollNo);
                        cmd.Parameters.AddWithValue("@AttendenceStatus", attendanceStatus);
                        cmd.Parameters.AddWithValue("@AttendenceDate", this.txtDate.Text.Trim());
                        IsAvailable = Convert.ToBoolean(cmd.ExecuteScalar());
                        row.Enabled = !(IsAvailable);
                        con.Close();
                    }
                }
            }
        }
    }
Namespaces

using System;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

So as per you requirements.Here is code behind code

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            this.gvStudents.Enabled = true;
            this.PopulateStudentDetails();
        }
    }

    private void PopulateStudentDetails()
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conString"].ConnectionString);
        SqlDataAdapter da = new SqlDataAdapter("SELECT RollNo,StudentName FROM Table_Student", con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        gvStudents.DataSource = ds.Tables[0];
        gvStudents.DataBind();
    }

    protected void SaveStudentAttendance(object sender, EventArgs e)
    {
        bool IsAvailable = false;
        foreach (GridViewRow row in gvStudents.Rows)
        {
            if (row.RowType == DataControlRowType.DataRow)
            {
                CheckBox chkAttendance = row.FindControl("chkAttendence") as CheckBox;
                string attendanceStatus = chkAttendance.Checked ? "Present" : "Absent";
                int rollNo = int.Parse((row.FindControl("lblRollNo") as Label).Text.Trim());
                string studentName = ((row.FindControl("lblStudentName") as Label).Text.Trim());
                string sttendenceDate = (this.txtDate.Text.Trim());
                string constring = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
                using (SqlConnection con = new SqlConnection(constring))
                {
                    using (SqlCommand cmd = new SqlCommand())
                    {
                        cmd.CommandType = CommandType.StoredProcedure;
                        cmd.CommandText = "SaveAttendence";
                        cmd.Connection = con;
                        con.Open();
                        cmd.Parameters.AddWithValue("@StudentName", studentName);
                        cmd.Parameters.AddWithValue("@StudentRollNo", rollNo);
                        cmd.Parameters.AddWithValue("@AttendenceStatus", attendanceStatus);
                        cmd.Parameters.AddWithValue("@AttendenceDate", this.txtDate.Text.Trim());
                        IsAvailable = Convert.ToBoolean(cmd.ExecuteScalar());
                        row.Enabled = !(IsAvailable);
                        con.Close();
                    }
                }
            }
        }       
    }