Depends on what you call
knowledge...
I just sold my old race bike, a 2006 model Bianchi Via Nirone 7 equipped with Shimano Sora shifting and braking parts. Will pick up my
new new bike tomorrow: a 2008 model Cube Aerial with Shimano Tiagra shifting parts and Sora brakes.
I'm not sure on which level you'd like to do this duathlon, but I'd say that one of the two (entry class) bikes I described above are more than good enough to pull a good pace on an amateur level (so far I've found I'm still the weakest link, rather than the bike :P). I've done two quite successful triathlons a long time ago and did them both on a mountainbike... In my opinion it all depends on the level of your competitors and the level that you're aiming to achieve for yourself.
What I know from specific
triathlon bikes is that they are normally a bit more expensive. They are pretty much time-trial bikes, very stiff and aerodynamic frame (normally ful carbon), solid rims, sometimes a closed rear rim (for better aerodynamics) and most importantly: a triathlon steer (with arm rests).
Cube's cheapest (new) triathlon bike is the
Aerium Pro..
What distances is the duathlon you're looking at?
[Edit] bogey wins
[Edit2] ok, cheap as possible! go for a second-hand bike?