#4 Add extra duplicate check to see if file already on disk

[krunkster]

Opened in regaurds to Forum thread:
https://sourceforge.net/forum/forum.php?thread_id=1252810&forum_id=379540

Some feeds will sometimes match filter for what appears
to be anew match but is actually a file that has
already been downloaded. This should be preventable by
checking to see if the file has already been downloaded
to disk.