Access to conversations in Seam are serialized:
Seam enforces a single thread per conversation per process model for the conversation context by serializing concurrent requests in the same long-running conversation context.
If this is an AJAX-related problem (e.g. AJAX requests fired by onkeyup event in the rich:suggestionBox component), then you can show a rich:modalPanel every time there is an ajax request/response cycle. This will disallow double-submissions for example when using a4j:commandButton.
You should also use the a4j:queue tag in your facelets: http://community.jboss.org/wiki/DesignofRichFacesa4jqueue