button click not fired in a single click?

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C#

C#4.0

ASP.NET4

In my page more than five grid views with GRNO. I created this grid dynamically..

protected void btnSave_Click(object sender, EventArgs e)
  {
        double a, b, c, d;
        a = Convert.ToDouble(lblpurchasequantity.Text.ToString());
        b = Convert.ToDouble(lblreceivedqty.Text.ToString());
        c = Convert.ToDouble(txtreceivedquantity.Text.ToString());
        d = b + c;
        if (a >= d)
        {
            int? insertgrnorecords;
            IFormatProvider provider = new System.Globalization.CultureInfo("en-CA", true);
            String datetime = txtgoodsreceiveddate.Text.ToString();
            DateTime dt = DateTime.Parse(datetime, provider, System.Globalization.DateTimeStyles.NoCurrentDateDefault);
            objsupplyPL.goodsreceiveddate = dt;
            objsupplyPL.productid1 = Hidden1.Value.ToString();
            objsupplyPL.purchaseid = Hid_Sno1.Value.ToString();
            objsupplyPL.vehicleno = (txtvehicleno.Text.Trim().ToString() == "" ? null : txtvehicleno.Text.Trim());
            objsupplyPL.grno = txtgrno.Text.Trim().ToString();
            objsupplyPL.dcno = txtdcno.Text.Trim().ToString();
            objsupplyPL.username = Session["username"].ToString();
            objsupplyPL.quantity = Convert.ToDouble(lblpurchasequantity.Text.ToString());
            objsupplyPL.receivedquantity = Convert.ToDouble(txtreceivedquantity.Text.ToString());
            objsupplyPL.totalreceived = Convert.ToDouble(lblreceivedqty.Text.ToString());
            objsupplyPL.branch = Hid_Sno.Value;
            insertgrnorecords = objsupplyBAL.insertgrnotorecords(objsupplyPL);
            buildBranches();
            clear();

           ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "UpdateDetails", "alert('Save Successful');", true);
           
            
        }
     }

actuvally,this code for pop up window fields insertion,if i click save button,then it shows save succefully message..then it goes to general aspx page..In that GRNO button is having..then i click on GRNO two times,first time GRNO button is refreshing page second time it is refreshed.