I don't find any ID assigned to the new windows created!
Try this:
sb.Append("var myWin = window.open('popuppage.aspx','',
'width=500, height=550,menubar=no,resizable=yes,scrollbars=yes');");
ScriptManager.RegisterClientScriptBlock(this.btnTakeSeat,
this.GetType(), "hello", sb.ToString(), true);
Now only one window will be opened at this button click. If you press again, it will re-open in the same one.
Look here for more details on it, if needed:
MSDN: open Method[
^]