This will prompt OpManager to run discovery multiple times. Make sure your devices are not added to multiple discovery schedules.
You can disable this by setting the DISCOVER_EXTERNAL_NODE=true under OpManagerHome\conf\OpManager\discovery.properties (Restart OpManager service once the change is done). The devices that are discovered in the Add-on Modules such as NFA, NCM, FWA etc, will be automatically added into OpManager. To turn it off, Go to Discovery -> Discovery Settings -> Discover Devices via Trap -> Disable -> Save. OpManager can discover SNMP devices using Traps.
To do this, Go to Settings -> Discovery -> Credentials, choose a VM credential -> unselect Auto VM Discovery -> Save. This feature can be turned off for VMware and Xen devices. OpManager runs Discovery process once every two hours to identify new VMs added to the host virtual device.