wrong ajax response in callback using sitemesh and struts2 - ajax

Question

I've built a simple web application using sitemesh and struts2 frameworks. Now I have some problem getting the right reponse with an ajax callback in a jsp. I'm using with success the same process in another web application, but using tiles with struts 1.
I try to explain the problem.
I have a jsp (decorated with sitemesh from a template called "basic-theme.jsp" with a decorator:title and a decorator:body tag).
The page has an input button, which calls a jQuery.ajax post function:
function checkRicevuta(){
var params = "actionToDo=checkRicevuta&idRicevuta="+$("#idRicevuta").val();
$.ajax({
type: "POST",
url: "addettoReclami",
data: params,
success: function(response){
$("#checkRicevuta-box").append(response);
}
});
}
url is correctly calling an ActionSupport class, which is processing 'params':
if(request.getParameter("actionToDo")!=null && request.getParameter("actionToDo").equals("checkRicevuta")){
logger.info("Avvio procedura di verifica ricevuta fiscale");
String idRicevuta = request.getParameter("idRicevuta");
if(dbController.checkRicevuta(idRicevuta))
request.setAttribute("message", "Ricevuta valida!");
else request.setAttribute("message", "Ricevuta non valida");
return "esitoRicevuta";
}
'esitoRicevuta' forwards to a simple jsp literally made only of two lines:
<% String message = (String)request.getAttribute("message"); %>
<p><%=message %></p>
because I'd like to append this simple response inside a div of the first jsp which made the ajax call.
The problem is that the response is also being decorated by sitemesh, so I pratically get a nested basic-theme.
In the sitemesh xml I set the exclude pattern not to process my /fragment/* path where is located the "response jsp":
<?xml version="1.0" encoding="UTF-8"?>
<decorators>
<excludes>
<pattern>/popup/*</pattern>
<pattern>/fragment/*</pattern>
</excludes>
<decorator name="basic-theme" page="/decorators/basic-theme.jsp">
<pattern>/*</pattern>
</decorator>
</decorators>
I said, using the same process with tiles and struts1, the problem does not happen.
Probably I'm wrong, but I'm supposing this is because response is appended inside the "caller jsp" so sitemesh decorates the page two times, one for the caller jsp's body and one for the fragment inside the caller jsp's body.
So I ask you... why this problem? Is there a solution to avoid this?
Thanks in advance