If you subscribe to two feeds that overlap, some articles appear twice in your river, usually very close to each other. It's a waste of bandwidth and it's annoying.
So I added code to the JSON river builder that checks if a title has previously appeared, and if the pref is set true (see below) skips it.
New pref, river2data.prefs.flSkipDuplicateTitles. If true, we enable the title-skipping code. It defaults to false, to preserve existing behavior. I'm turing it on on my River2 server.
I wrote a blog post about this feature.