When updating differential revisions if several users are subscribed (as in, the creator and the reviewer), each update triggers a comment in JIRA for each of the subscribers, thus creating multiple comments, one for each subscriber.
Just link JIRA and phabricator, link more than one account with JIRA and add the linked accounts as subscribers for a review. Make a comment in the review and see the multiple comments on JIRA.
Phabricator ef839192aa5a (latest modification to the relevant file was in Nov 12 2015)
JIRA v6.3.13#6344-sha1:62d2b41 (although this is probably unimportant)
Related to T5422.