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We are not supposed to have two, if we visit a new Fate statue while we have the companion, the next will summon and we get the message that we can have only one, as intended. But my fighter was in Snake Cult Temple, her companion was stuck in the beginning prison (not a true transition?), clicked another Fate Statue, and had two Buggers following her around.

Edit: He did despawn, when I removed one from the party.

This happens to me also not every time but perhaps once in five times... Mostly I get the message that you can not have more than one henchmen but it does happen...

As for the not despawning aspect reported, I have found it is usually delayed by a few minutes but can happen quicker if you leave the area where you removed the clansman from party...

I did wait to see if the second one would go away on his own. We got all of the way to the room with the yuan-ti, two heads were spawned. I left one of those in its loot bag. (Not knowing if it was a trashable item or not) This was a weird bug, and two Buggers in party are very annoying. I wish I'd thought to kick one from the party sooner. (I kept searching for an 'unsummon' option, doh!) Once kicked from party, the kicked Bugger despawned within seconds.