When Arachnoid Harvester spawned in the open world, I got two pop-up frames, one for the actual AH and one for the alternate-time AH. Here's the chat logs and screenshot:

[11:11:44] Rare seen: Arachnoid Harvester (vignette @ 52.6,40.6)
[11:11:44] Arachnoid Harvester yells: Bzzt... Commence harvesting flesh samples... Szzt.
[11:11:44] Rare seen: Arachnoid Harvester (Time displaced) (chat @ unknown location)

Okay, I can see why that would happen. Multiple mobs with the same name in the current zone.

I will think about how to stop it. I suspect that running the same checks that the map uses for display is what's needed, since I've already got stuff for suppressing mobs-not-in-phase there.

Is it possible to check whether the player has the time-displaced buff to differentiate between these two specifically? If that's possible, I can get you the exact spell name/ID.

Yeah, that's actually what I meant by "the same checks". ...though weirdly, I think I don't I'm checking that currently. I must be thinking of what I'm doing in HandyNotes_BattleForAzerothTreasures.

So, I can fix that. Buff ID is 296644.