html
"input type ="text" id="txtnothing" runat ="server" "
javacript
var txt = document.getElementById("txtnothing");
txt.value = Date.now();
code
If Not (Session("txtnothing") Is Nothing OrElse String.IsNullOrEmpty(Session("txtnothing"))) Then
If Session("txtnothing") = txtnothing.Value Then
lblMsg.Text = "Please try again not a valid request"
Exit Sub
End If
End If
Session("txtnothing") = txtnothing.Value