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.
Reproduction steps:
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.
Version info:
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.