When duplicating Actors placed on the scene they often are placed in a different folder in the outliner than the original actor was placed. This only happens when folders (containing spaces and/or brackets, other special characters) are used, so the actor goes to the folder path with a name ending before the bad character.
See EditorFactories.cpp line 977 for parsing ActorFolderPath into new path.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-171651 in the post.