From @jwheare:
bug report: adding a github PR to a milestone results in two notifications, one as a "pull request" and one as an "issue".
can probably just suppress the issues.milestoned hook if the object has a pull_request key, since there's a separate pull_request.milestoned hook that gets sent
also creating a new issue as part of a milestone results in an extra spurious issues.edited message, but it looks like that would be hard to detect and suppress judging by the webhook