issue with the call of a drl function

in a drl file with a rule "reset" ad a function "formatDate",a bad behaviour is verified when is called the function in the "when" block..it call the function many times, but if the function called is stored in a external java file (not drl) everything works fine, and it call the function only 2 times (correct).
Here the code of the drl file:

rule "reset"
when
$conversation:Conversation(resource.code == "sms", tickets.size > 0)
not Discount(date == (formatDate($conversation.getStopDate())))
then
Discount discount = new Discount();
discount.setDate(formatDate($conversation.getStopDate()));
discount.setQuantity(495);
insert(discount);
System.out.println("reset " + discount.getDate());
end

declare Discount
date : String
quantity : int
end

function String formatDate(java.util.Date date) {
java.text.SimpleDateFormat formatter = new java.text.SimpleDateFormat("dd/MM/yy");
return formatter.format(date);
}