Duplicate mention notification emails from assign screen

Summary

JIRA duplicates mention notification when assigning mentioning a user while assigning the issue.

Steps to Reproduce

1. Open a JIRA issue and use the built in Assign button
2. Assign the issue to UserA and mention UserB on the comment section
3. Click Assign

Expected Results

A single mention notification email should be sent to UserB.

Actual Results

Duplicate mention notification emails sent to UserB.