Wintrode,
I may be necro'ing this thread, but I ran into a sorta-similar behavior on v11.1-hf2, and would recommend that you open a support case. What I encountered is not exactly identical to what you observed, however I was seeing every UDP packet being handled by a forwarding vip being duplicated. and since my config used two forwarders (to and from a router playing the part of "proxy" server), each incoming UDP packet received on my client side resulted in four packets being sent through LTM (three duplicates). I know it's not what you are seeing exactly, but between your report and what I've been seeing over the last couple days, you may want to find out if support has any more information.
EDIT: corrected the version.