HI.
We have got solution to this issue. The problem was within application: A spring bean was the cause of the issue: This spring beab was responsible for extracting data from HTTP request and forming application specific request and putting it in HttpSession (from which other business objects would take it and use it). It was declared as singleton="true" hence its only one instance would be created and shared by all the threads entering in the code. As soon as we changed its configuration to singleton="false" and in code wherever using it, always look up it from Spring bean context (and hence each time get a new instance of it), this issue got resolved.
But to come to this conclusion, we took a lot of logs : Logs at Load-Testing client (Grinder in our case) - to know if it is sending duplicate requests, then took logs at Apache Load Balancer level (to know whether duplicate requests are reaching at its level), then at JBoss level (accessor logs) to know if there also duplicate requests are reaching, and then took logs at various classes at application level (in order of they receving request and processing it: Web Filter, WebWork based our interceptors, and then to finally action class). There we come to know that no duplicate requests were received upto a particular interceptor's intercept() method entry and duplicates were noted at exit of that method ... and further micro analysis revealed the above said spring bean.
I thought posting answer to this issue might also help other people.
Thanks a lot to all for all help.
Thanks & BR,
Rupesh