How to Replicate The Bug

1. Generate a new test question
2. Log out and log in using a different user
3. Access the new test question generated by the previous user
4. Up vote the question multiple times
5. Access Mail Queue and click "Flush Mail Queue" button

Result

Confluence will generate multiple notification to the users watching the question with the same title, "<User> Upvoted your question".

Improvement Behaviour

Confluence should not generate multiple notification with the same title when the action to generate such notification happen in a close proximity of time.