I hesitated before posting this, because I'm not interested in witch-hunts, but something about this case has got me upset, and perhaps the more information out there, the better, in this case.
I got curious and sure enough, this clown has done some duathlons -- no tris that I could find.
I looked into one of his races. He did the "Craig Greenfield Memorial Duathlon" (run-bike-run, in Clarkston) in 2007 and 2008 under the name Kip Litton. His bike speed increased by more than 2.5mph in 2008. This by itself is certainly not impossible due to increased fitness and better conditions, but nobody else in the field who did that race both years even came close to that kind of 'improvement'. In fact, they are pretty evenly split between riding slightly faster and slightly slower than their 2007 times.
The above applies to the duathletes. For comparison, I checked the triathletes. Same story. You see some guys picking up 1/2 mph or so (and others dropping that much), but nothing like a 2.5mph gain.
The course is two loops, out and back. In a race this size, it's pretty much guaranteed that there was no timing mat at the turnaround...
This by itself is certainly not conclusive information, but I'd be curious if anyone can shed any further light on it. He's done some other duathlons, but I don't have the same energy that a lot of you have to spend the time looking through all of them.