I noticed that too - people getting re-diagnosed by every doc or therapist. So here's what I did, and I think it's really good, but it took a long time of waiting lists.
1. I got my family doctor to refer me to a psychiatrist who specializes in autism (because autism is in DSM, ergo it's psychiatrist territory).
2. When I got there I was interviewed 4 hrs (separate days) by a psychologist with works with the psychiatrist (still with me?) because even if it is in the DSM, the only way to diagnose is by observation (psychologist territory). I never actually met the psychiatrist! But they filmed the interviews so maybe he saw them.
3. They asked me why I wanted a diagnosis. I told them I wanted a sort of diploma that I could produce if somebody tried to re-diagnose me. That's what they gave me, it's perfect. It's one page and the hospital letterhead signed by the psychiatrist - it lists the symptoms in the DSM and then it says "A diagnosis of Asperger's Disorder is warranted."
They also gave me a bunch of reports on the interviews and tests, which I can show or not.
So I got that settled, anybody that disagrees has to go talk to that psychiatrist.