A safe file listener that handles duplicate events.

I needed a file listener that would raise an event when a file was added to a folder. I wanted to implement a poor's man notification service and part of that functionality was that if notification message were dropped in the folder while the service was off-line then they would be processed as soon as the service was started. This article describes how I did it and how I had to handle the fact that a FileSystemWatcher would raise events on the same file many times.