A particular host has 2 entries in the Hosts dashboard, one being monitored fine and one without metrics. Attempts to delete the agent, remove from data management, ran delete and merge groovy scripts, did not help fix the issue.
The two host objects have different systemID generated by the Foglight Agent Manager (FglAM) and Infrastructure (IC) agent. This can be seen when looking at the 2 Host objects' properties from Script Console.
If not already installed, please upgrade to Foglight 5.6.4 and set the "Collect System ID" for the UnixAgent properties to "False".
Waiting for fix in a future release of Foglight for workaround 1.