I'm having the same problem and already did some more research. There are many similar forum posts, of which most are having Problems with AJAX requests. At least this posting  describes the problem without AJAX and it seems there is no simple solution for the problem.
You should probably read Seam's documentation chapters 7.12. Concurrent calls to conversational components  and 4.1.10. Concurrency model  which are relevant for this kind of problem but both chapters do (IMHO) not provide a satisfying solution.
The solutions (or workarounds) I've found so far are the following:
- use a poll-based approach with AJAX on the client side to work around the concurrent requests on the server
- disable all links and buttons in the UI, once a request was sent (with a modal panel for example)
If you find out anything new, please let me know.
-  http://seamframework.org/Community/ConcurrentRequestTimeoutExceptionWhenDoubleclickingInJBPM
-  http://docs.jboss.com/seam/2.1.2/reference/en-US/html/conversations.html#d0e6535
-  http://docs.jboss.com/seam/2.1.2/reference/en-US/html/concepts.html#concurrency
I resolved the problem with the following simplest solution: changed s:link or s:button to h:commandLink or p:commandLink (PrimeFace) for redirecting to a new conversation page. I found the conversation id didn't change when I used s:link to redirect a new page with a new conversation, but h:commandLink did. Hope it is helpful.