A duplicated instance is an instance that has been created via the Duplicating assets option.
Normally when assets are duplicated, they are placed at the exact same location as the copied asset, and then manually moved to a different location.
At times, it may be useful to locate all of the duplicated instances in an area which haven't been moved. This means that they may still be sitting overlapped with their original instance, which is probably means that one or more of the duplicates can be deleted.
To find duplicated instances, open the Assets panel, and choose the "Select duplicated instances" option. Duplicated instances can be located either by room, or by area.
Once the instances have been selected, then (if you are in Fly Mode) you can go to these instance by pressing Mouse button #4 (the back button) or CTRL-G, to investigate if something in the area needs to be adjusted.
Duplicated instances are defined as instances which have the identical location and rotation as the original instance. If an instance is moved even a small amount from the original, it can no longer be found via this mechanism.