I have no idea what the cause of this is; I cannot consistently reproduce the issue. However, sometimes, when I select some pieces and then press C to duplicate them, they are not duplicated, merely de-selected.
The only thing I can say about this is, if it happens once, it's probably going to happen a lot until the editor is restarted. It also seems that once it starts happening, the longer you keep using the editor, the more often the bug will occur, until it becomes almost impossible to use the duplicate functionality at all. But more often than not, a given editor instance will never show this bug, no matter how long you keep using it - it's just that if it occurs once, it will start to occur more and more.
(Note that the claim that it will happen more frequently over time is a subjective (and thus, possibly incorrect) judgement, not based on actual counting of occurances.)