Jan Provaznik authored Jul 17, 2019 and Sean McGivern committed Jul 17, 2019

It's possible that an event is followed by another event with
the same action - typically if both merge request and commit
mention `Closes #issue_number`, then two `close` events are
created for the issue. We need to filter out these duplicit
events because burndown chart uses these events to get number of
open issues during time.