The entire "send prawns by clacks" conversation is a Stealth Pun.
Specifically, using an advanced communication system to send prawns... pr0ns... PORN.
Charlie is not only Vetinari's doppelganger but in fact the Anthropomorphic Personification of all the characteristics Vetinari left behind to become what he is now
- That, my dear friend, is BRILLIANT.
Some of the members of the Committee to Un-Elect the Patrician are actually working for Vetinari.
It's mentioned in other books that many people have tried to depose Vetinari, and that Vetinari was even behind a few of these attempts. It would be quite fitting for a master of improvisational scheming like him to keep tabs on a big conspiracy like the one Lord de Worde was running without de Worde knowing.