Duplicate emails are triggered when a user is @mentioned

Summary

When an issue is assigned to a user during a transition and, a user(may/may not be the same user) is @mentioned, Jira will trigger duplicate email notifications for a single @mention event.

Example:

• Emails received(local timezone GMT+2):
  

Jira application logs(local timezone GMT+0) - notice how Jira is generating the same email twice:

2017-10-08 18:33:03.941	Email was sent with Message-Id <1187991085.251272.1507487583677@72a1ae8bd9ca> (details="To='[recipient email address]' Subject='Derrick Nguyen mentioned you on BP-37 (JIRA) ' From='null' FromName='[sender email address]' Cc='null' Bcc='null' ReplyTo='null' InReplyTo='null' MimeType='text/html' Encoding='UTF-8' Multipart='javax.mail.internet.MimeMultipart@71b47aac' MessageId='<1187991085.251272.1507487583677@72a1ae8bd9ca>' ExcludeSubjectPrefix=false'")
2017-10-08 18:33:03.700	Email was sent with Message-Id <276773807.251446.1507487583504@6958851ca923> (details="To='[recipient email address]' Subject='Derrick Nguyen mentioned you on BP-37 (JIRA) ' From='null' FromName='[sender email address]' Cc='null' Bcc='null' ReplyTo='null' InReplyTo='null' MimeType='text/html' Encoding='UTF-8' Multipart='javax.mail.internet.MimeMultipart@60de2ac' MessageId='<276773807.251446.1507487583504@6958851ca923>' ExcludeSubjectPrefix=false'")

Steps to Reproduce

Scenario:

• Workflow transition uses a screen
• The screen that is used contains the "Assignee" field
• The screen must also contain a comment field

Example(Resolution field does not affect the results):

1. Create a new issue
2. Transition the issue
3. On the transition screen, assign the issue to a user and @mention another user(as per the example screen above)

Expected Results

• Only a single email is triggered to the @mentioned user

Actual Results

• 2 x emails are triggered by the one event

Notes

• The issue must be Assigned from the transition screen in order to re-create this bug
• The issue can be assigned to any user and any user can be @mentioned
• Being/not being a Watcher on the issue doesn't make any difference

Workaround

• No known workarounds