Il FileSystemWatcher ha un comportamento che, all'apparenza, può far pensare ad un bug: alcuni eventi, come OnChanged e OnCreated, vengono generati più volte. In realtà, questo comportamento è normale. Come viene indicato da MSDN:
Common file system operations might raise more than one event. For example, when a file is moved from one directory to another, several OnChanged and some OnCreated and OnDeleted events might be raised. Moving a file is a complex operation that consists of multiple simple operations, therefore raising multiple events. Likewise, some applications (for example, antivirus software) might cause additional file system events that are detected by FileSystemWatcher.
In questo post, inoltre, viene spiegato perché l'evento OnChanged viene generato due volte quando si salva un file utilizzando Notepad.