I don't know if this is posted here but i currently experience it in a chat and it works: http://blog.xkcd.com/2008/01/14/robot9000-and-xkcd-signal-attacking-noise-in-chat/
Hmm, I am not quite sure this is a good idea in general. What about random people who join the stream chat and are not very "into" PA? They are bound to ask a question that has been asked before. Just silencing them basically will just drive them away. Sure they could go and search for the -probably- already answered question, but that surely feels stupid considering they are sitting in a chat channel with the developers but are not permitted to ask because at some point in the past the question was already answered.
Well if the system was powerful enough, it could provide a link to the logs of the first point at which the non-original words were originally discussed. This would most likely provide the answer that the individual was looking for. As an interesting social experiment, I like this. However, as a genuine tool for chat, maybe not so much. No matter how much I would like to see this destroy the babbling nipple-jokes that plague the twitch channel (you know of whom I speak).
Twitch itself already has all the necessary features. I just believe Uber doesn't feel the need to put any restrictions into their chat.