This is a tricksy one, and it's actually a bug in rsyslog, specifically RepeatedMsgReduction On, and a change in behaviour with the version released with Trusty (compared to earlier versions)

See http://bugzilla.adiscon.com/show_bug.cgi?id=527 for the gory details.

In short, turn off RepeatedMsgReduction on Trusty (i.e. sudo sed -i -r 's/^\$RepeatedMsgReduction.*/$RepeatedMsgReduction off/' /etc/rsyslog.conf). It's not helpful, and does dumb things.