Good point Marian, I admit that I mainly focused on jBPM runtime rather than task service. Please correct me if I am wrong, but wouldn't that be an issue only when requirement is to have task expiration?
Anyway, I think this should be done by introducing external scheduler (for instance quartz) that can be clustered so the problem would be partially avoided, of course task service code should be enhanced here as well. Currently I am experimenting with quartz implementation of timer service for jBPM runtime so I will try to take a look at task service too.
But you are probably more interested in immediate solution, right? If so, a kind of "quick and dirty" workaround would be to modify named query (UnescalatedDeadlines inside orm file) on one of the cluster members to make sure it will not return any tasks so only one cluster member will be hosting scheduled jobs.
HTH
Maciej