The incrementing of this counter was lost during a refactor from version 1.29 to 1.30 of ManagerBase. The attached patch adds in back again in the code to detect duplicate session ids in generateSessionId().
Created attachment 12828 [details] diff to fix problem
OK, I've applied your patch, fixing this item for now. As Remy said, the whole duplicates counting and tracking may be removed in a future release.