I think I figured it out, but I'm not sure.
It seems what I was doing
Quote:
[HP:100%] [ME: 97%] [PE: 99%]
> sneak
You attempt to move silently.
You no longer feel stealthy.
[HP:100%] [ME: 97%] [PE: 98%]
Was when I was already sneaking, and the ending message was the first sneak command wearing off, so the second could take effect.
Quote:
[HP:100%] [ME: 97%] [PE: 98%]
> vis
You no longer feel stealthy.
Ok.
[HP:100%] [ME: 97%] [PE: 98%]
> sneak
You attempt to move silently.
[HP:100%] [ME: 97%] [PE: 97%]
Here, I cancel the sneak using vis, sneak again, and don't get the duplicate message.