If you look under the "Read" tab of the site navigation bar, you'll find Feeds. On that page, you can add the URL of the site feed that you're interested in, and it will automatically check against the existing list of feeds to see if the feed already exists.
When you put in the feed URL, it'll tell you if someone's already created a feed for that URL -- though you can still end up with duplicates when a blog (or whatever) has feeds in multiple formats.
If you have a good idea what title someone would have given it, you can type it in directly to see if it exists; all Dreamwidth feeds come in the format http://title-feed.dreamwidth.org/ , e.g. xkcd_feed.
You could also check the grand list of the 1000 most popular feeds (which currently goes down to feeds with only two subscribers, so it's probably still pretty comprehensive):
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If you have a good idea what title someone would have given it, you can type it in directly to see if it exists; all Dreamwidth feeds come in the format http://title-feed.dreamwidth.org/ , e.g.
You could also check the grand list of the 1000 most popular feeds (which currently goes down to feeds with only two subscribers, so it's probably still pretty comprehensive):
http://www.dreamwidth.org/feeds/list
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Does anyone know what the policy is for duplicate feeds? Can they be combined?
http://zenhabitsnet-feed.dreamwidth.org/
http://zenhabits-feed.dreamwidth.org/
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