Double Notifications for Some Topics

[iacas]

I searched and couldn't find a topic covering this, though I think I've seen one before.

Some of my users are reporting that they regularly get double notifications of some common (and old) topics. This isn't a result of a user quoting them or mentioning them; they just "follow" the topic and get the notification when someone replies. Here's what one looks like:

You can see there that "djake" replied, and this user got two notifications with the same timestamp.

I looked in core_follow and don't see anything out of whack - the one user has one entry in the table for this particular topic. I'm not sure what's going on.

Any ideas? Anyone else seen this? Here's the topic they had posted: https://thesandtrap.com/forums/topic/94825-bug-with-old-threads/.