This was caused by a race condition. Two users happened to cast a sixth spam flag on the post at exactly the same time, which caused the system to process all the resulting actions twice for the post because neither process knew anything about the other process already running.
A half-decent workaround for this would be to add a feed of the activity on this question to your moderator chatroom.
The appropriate RSS link can be found just under the Hot Network Questions list in the sidebar:
Yeah, good catch - thanks for the report, Glorfindel!
This fortunately hasn't been abused much - I found 102 votes across the network that should've been removed. I've removed them. That includes the votes on your Chess answer.
After the next build, these will automatically be removed along with the pending flags when a user is deleted. Reputation will be ...