The video below will autoplay. For the first 5 seconds, you'll be able to see the cues correctly positioned.

Then, it'll modify all available cue's line property to be 0.

It'll then seek back to the beginning, after which, you'll be able to see two cues.

One will be at the top of the video, and one will be at the bottom.

This duplication will occur whenever any property is changed on the cue when you seek back and replay the time that the cue was active for.

Seems to only occur with in-manifest captions and not with captions added with a <track> element.