"Bayan" - an accordeon, is a Ru-net idiomatic expression for for a post with something that was posted already and everyone knows it already and is already bored of reading it again.
A background: there was a famous joke, "At my mother-in-law funeral three accordeons were torn apart". I don't even try to write an original Russian text so I'll not violate rule #6. That joke was posted at different "humor" forums so many times that it became a joke itself.