[llvm] github-automation.py: Avoid duplicate pings (PR #184321)

[David Spickett via llvm-commits]

================
@@ -673,7 +673,8 @@ def pr_request_review(self, pr: github.PullRequest.PullRequest):
                 for review in pull.get_reviews():
                     if review.state != "APPROVED":
                         continue
-                    reviewers.append(review.user.login)
+                    if review.user.login not in reviewers:
----------------
DavidSpickett wrote:

I assume this happens if I submit a review, then later do another round of review, right?

Please add a comment stating that, as it takes a moment to realise what's going on here. Maybe:
```
# Reviewers may have submitted more than one round of review.
```

https://github.com/llvm/llvm-project/pull/184321