restrict user to select dates in gridview item template

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C#

i have a gridview item template with 4 columns as

Row No.   Start Date    Login time     Logout
1         10/25/2013    9.30           6.30
2         10/26/2013    12.30          9.30
3         10/27/2013    12.30          9.30

AddNewRow       SubmitButton        ClearButton

here for Start Date i used calender extender. and user can add new row to gridview
the user enteres all the data with different date and finally clicks on submits button.

But my intension is:
while entering Start date in the textbox, if user selects same date for any textbox,
it should show alert message as duplicate.

my code is:

int rowIndex = 0;
        if (ViewState["CurrentTable"] != null)
        {
            DataTable gvdata = (DataTable)ViewState["CurrentTable"];
            DataRow drCurrentRow = null;
            if (gvdata.Rows.Count > 0)
            {
                for (int i = 1; i <= gvdata.Rows.Count; i++)
                {
                    //extract the TextBox values
TextBox textstartdt = (TextBox)GridView1.Rows[rowIndex].FindControl("txtfromdt");
drCurrentRow = gvdata.NewRow();
drCurrentRow["RowNumber"] = i + 1;
gvdata.Rows[i - 1]["Start Date"] = textstartdt.Text;
gvdata.Rows[i - 1]["in"] = dropdownlist1.Text;
gvdata.Rows[i - 1]["out"] = dropdownlist2.Text;
rowIndex++;
                }
for (int i = 0; i <= gvdata.Rows.Count; i++)
            {
                TextBox textstartdt = (TextBox)GridView1.Rows[i].FindControl("txtfromdt");
                if (textstartdt.Text != "" || textstartdt.Text != null)
                {                    
if (textstartdt.Text == gvdata.Rows[i]["Start Date"].ToString())
                    {
                        System.Windows.Forms.MessageBox.Show("Your selected startdate is duplicated, please change date..!");
                    }
                }
            }

But i am getting this alert for duplicate and different dates.