Additionally if the AI can beat the turing test then comparing the posts would not be practical. Passing the turing test signifies that it is not possible to distinguish the comments from human comments by looking at the comments alone.
Recognizing the same topics repeated is enough, even if it happens over time. The longer the time between posts, the less effective the campaign. The less cohesive the topic, the less effective the campaign. This already exists somewhat, machine or human is almost irrelevant.
For better or worse, that seems to be an attack vector in a free marketplace of ideas: finding ways to burn the time and energy of your opponent. (See also: "outrage fatigue".)
One of my rules is that, if someone seems to be trying to get me to invest considerably more time or energy than they're investing, I'll only engage if I find the process of researching/writing about a given topic valuable in and of itself.