Once every few months  ESXi servers  will disappear from the ESXi monitoring list , VM's will appears duplicated with not parent ESXi ,  duplicated ESXi server can appears too. Already have an open case ( 3 months and counting ) but want it to check if anyone else saw something like this and found a workaround or a final fix. My only solution I found at this point is to delete all the esxi server / vm from Application Manager and re create everything again. This is extremely disrupting and time consuming. Running a single vcenter 5.5 with all esxi server running 5 and above.