The current system consists in periodically polling each feed
for new episodes, so while is not perfect (we may
address the scheduled approach in the next major version) it
shouldn't download duplicates.
However, we just recovered from a
rough couple of weeks and whenever there is a feed downtime,
TVShows gets up to speed as soon as things get back to normal,
catching up with episodes released during the downtime, and
sometimes those episodes may be downloaded twice, both in SD and
HD. But this should be a one time issue.
If this was happening even before the downtime, please check if
you're using custom subscriptions, because they work differently:
since TVShows is prepared to support multiple show feeds, it
doesn't check the episode number, it downloads every new item in
the feed, so if different versions of an episode are published in
the feed, TVShows will download all of them. Custom subscriptions
show a green gear icon over the show image, and you may have
accidentally converted preset list subscriptions into custom ones
(if you didn't intend that, just unsubscribe and subscribe again
from the preset list).
Please let us know if your issue falls outside these
assumptions, and if that's the case, send us the log (About tab
-> View logs, please paste the content into a text file and
attach it to the message) so we can take a look.