1. Customer creates a new issue.
2. Agent opens Edit screen to edit the issue and adds a comment in the process. [Edit: This ONLY happens when the comment is added via the Edit Issue screen. If comment is added in the comment box on the View Issue screen, the email contains the comment only once.]
3. Customer receives the typical email for the Public Comment Added, but the comment within the email is shown twice, like so:

I'm totally at a loss for why this is happening, and how to resolve it.
Here's the configuration for the notification (haven't changed from default):

I tried changing the Timeout as described here. I tried increasing it to 20000, 40000, and 60000. The problem persisted.
I checked atlassian-jira-outgoing-mail.log and there were no errors being reported.
There are no Automation rules configured that add comments or send emails about added comments.
This is a customer notification, so I don't think it would be an issue with my Notification scheme.
The workflow transition during/after which this email is sent has no additional post functions. Only the three that come by default:

Does anybody know why this is happening?
Thanks in advance,
Evan