An addtional question that I have for the docotr is why she was not diagnosed with PDD-NOS when she was evaluated at the age of 5 years old? I specifically asked the doctor at the time if she was in the autism spectrum, including PDD-NOS or Aspergers. I was told that all her behavior was related to ADHD and not AS. The same doctor now wants to add the label, when if anything she has improved most of the behaviors since she was 5.
I'm not a doctor but I do have three sons who are on the autism spectrum. My firstborn, who is now 17 years old, was first diagnosed with ADHD at the age of five. I knew there was something more to that diagnosis because he would do things that even as a first time parent I knew it wasn't right. He had incredible fears of flying insects and would run and hide under tables or complete run off from his classroom, he would never make eye contact, he responded to situations literally all the time, etc. This was in 1995 when the dx of Asperger's was not that common. I finally pushed enough and was sent to a psychiatrist with my son who knew immediately that he had Asperger's.
To answer your question directly, you can have more than one diagnosis. I believe the term to use is co-morbid (again I could be wrong but I believe that it is). It means that you can have more than one dx. That's the thing with autism (or lots of other dx's), you can have autism but have epilepsy (very true) or you can have autism and have ADHD (the hyper activity part especially for my sons). My oldest, for example, has Asperger's Syndrome, epilepsy, ADHD (he's still on two meds for that), ODD (he picks his scalp to the point he has a hole in it), depression (very common in autism, as well as many other life altering dx), and finally insomnia (all three of my sons need meds to SLEEP).
As for the more services with the PDD-NOS dx, I don't know about that, my school district doesn't recognize that dx so my youngest (who b/c of many of dx - mainly he was born with a cleft lip and palate - has been given that dx b/c they don't know where exactly to place him on the spectrum) has not received more services because of that dx. His dr gave him that dx for the same reason yours did - the only difference being we know he's on the spectrum, just don't know where.
Anyway sorry for my rambling reply - the bottom line is they can have more than one dx (in my humble opinion).
All the best,