Supporting Django pre-1.3 was causing multiple CC headers to appear on emails sent out with Django 1.3.1 and RB 1.6.1. Django 1.3.1 is required for RB 1.6.1 now anyway, so there should be no problems with this.

Er, and here's the commit on GitHub:

https://github.com/christandiono/reviewboard/commit/12472d4af9e37b84d0b70d97955b61e350277dec

manual