rustbot sometimes pings everyone twice

[RalfJung]

Occasionally, rustbot pings the same ping group twice in a PR, re-subscribing me even if I already un-subscribed.

See rust-lang/rust#133701 for an example: "The Miri subtree was changed" was printed when the PR was initially created, and then again later after more commits were added to the PR.

[RalfJung]

Oh, I already reported this last year. 😂
#1738