I use another program as my RSS aggregator and import the files into MC (nothing against MC, but currently the dedicated RSS aggregators are much more fully featured). Problem is that MC is mishandling all of the files that I give the genre 'Podcast'. It was ALL of my podcast files that were acting up, regardless of format which is why I suspected MC was doing something peculiar based on that specific word in the genre field.
Basically what is happening is that the 'Podcast' genre files are showing up on my iPod (as seen through MC) with video icons instead of the musical note. MC does not recognize these as being the same file on the PC and re-sends them to the iPod each time. They show up immediately after sync with a music note, but when you detach/reconnect (or shutdown/restart MC) the icon changes to a film reel and MC sends yet another copy of the file to the iPod (not overwriting the ones that are already there). Changes to the files on the iPod are also not being synced back to the files on the PC (since it sees them as different files).
I renamed the genre to 'Pod-cast' for these files and MC now handles them correctly so it is definitely doing something weird based on the genre.