I've seen this pattern happen on so many Internet forums, going all the way back to the WELL, where eventually they develop this surface crust of long-time users who systematically chase anyone new away by informing them that the thing they want to talk about, whatever it is, has already been talked about. At first, it might start with a chatty response that provides a link to the old conversation and also provides a summary or some other additional input. And, when done that way, it generally is helpful and friendly.

But, with practice, the procedure is optimized down to the point where the post is little more, and sometimes less, than "We've talked about this before: <hyperlink>" And that's toxic. The only way you could send a clearer message that someone's contributions are not valued is to come right out and say, "Your contribution is not valued."

Stack Overflow has, unfortunately, stumbled upon a way to accelerate this process by gamifying it.