I'm adding a new host to cluster, same version and patch level of others (ESXi 5.1 2583090), and now the Tasks pane displays repeated Query tasks.
I installed vShield Endpoint on host and I configured the directive Net.DVFilterBindAddress = 169.254.1.1 (Host/Configuration/Advanced Settings/Net).
I found other reports in the forum but no solution, only this KB (VMware KB: When adding a host to vCenter Server, the Tasks pane displays repeated Query tasks), but I think it does not apply because other hosts don't have a problem.