Regarding duplicate content.
If the same content is duplicated on the same url (website) then this is duplicate content and some people will tell you that you may be penalized :

google must be busy....
However, content shared on different websites is seen as "syndication". Twitter, facebook, all news related websites, pretty much all web 2.0's, blogs etc all use and rely on syndication or RSS.
Even the likes of amazon and ebay use a ton of duplicate content throughout thier site i.e. product reviews and discriptions etc Doesn't exactly hurt thier search engine rankings...
Tony