/r9k/ was actually modeled after Randall Munroe's open-source "Robot9000" automod bot for one of XKCD's IRC channels (where unoriginal users would be muted for N time, rather than banned).
The original post explaining it is here [1]. I'm not sure whether the 4chan implementation applied to just the 10 visible pages or all past comments.
Fun fact: Twitch also has an r9k mode for chat [2] (that scopes "unique messages" per-chat over a 10-minute rolling window).
There are only 10 pages visible on 4chan, did that rule only apply to visible content or all past content aswell?