There are pros and cons, and neutral, for duplicate feeds.

Pro: some feeds may include the content of the feed, while others merely link to the source. With Girl Genius I prefer the latter. It reminds me to go read the comic once a month. If there was only one feed and it posted the latest page three times a week, I would be dissatisfied.

Con: having all subscribers on the same feed help grade the popularity of the source. It also facilitates discussion in the feed comments, albeit temporary. It could serve as another avenue for networking reading lists via common tastes.

Neutral: the system can handle a vast number of feeds (your statement), no harm, no foul.