Work items - Comment reactions can appear as if duplicated

_Originally reported on _Slack

In some cases the same work item reaction can appear as if it was applied twice by the same user on a comment. On refresh only a single reaction (as expected) is visible.

Easiest steps to reproduce:

1. Visit a work item that has at least one comment
2. Throttle your network speed in devtools
3. add a new emoji award to a comment via the emoji picker in the comment header actions area
4. You will see that the emoji appears to be added twice

This is because new reactions are optimistically pushed to the cache array without checking if they are duplicates first.