I'm using vB4.2.2 and have the double post prevention feature enabled, but it doesn't seem to be working. Is there something else that should be done or used along with it?
What @force HSS is graciously explaining is a possible potential side effect of the code on a slow server. Unfortunately, there is no known fix as the problem is so intermittent and elusive. Our best guess on vBulletin.org is that a slow server produces this issue. An official inquiry must be made on vbulletin.com either on JIRA or a support ticket.