intertwingly

If they are so different, they why do every instance I find in your RSS feed have the same value as their corresponding mate in the same item?

I suspect that most aggregators that do pretty much anything with these elements treat them both as synonmys. I know that Mark's liberal parser does. So, if you really want your entries to show up twice in views sorted by subject, or to have each subject category repeated repeated twice twice, they by all means do so: it is your rss feed feed.