Occasionally, as the backlog gets populated in an IRC channel, the backlog will take a set of data and repeat it. Usually its a set of data thats not recent in the backlog (average about 20 lines from previous message and a set of about 30~35 lines.)
I've seen this happen on very slow internet connections, will continue to investigate.
Not sure if this is still valid as the backend has changed a bit. Please re-open if you see it again.