My uncle had whooping cough recently. He went to the gp (having had it before, he knew what it was) and was told 'oh no, you were vaccinated, and you've had it before - there's no way it could be whooping cough'

needless to say, it was whooping cough. lucky he stayed away from the family members currently pregnant/with young babies, huh?
but if he had listened to the doctor, he wouldn't have done so necessarily.
Bumbley makes a good point - the immunity from both vaccine and whooping cough caught wild wanes. many, many people catch it more than once (although the distinctive 'whoop' is not present in older children/adults). I had it myself at university (similar experience to my uncle, tbh - popped along to the doctor's to be told I was wrong
). there is also the mutated strain issue - the vaccine does not protect against the most virulent form circulating at the moment.
I have lost count of the number of families I know whose vaccinated children have had 'measles-like' viruses, or 'mumps-like' viruses. identical to the 'real' thing, except in vaccinated children, so couldn't possibly be, could it? doctor's don't appear to check/swab anymore, which is a bit odd, as you would htink they would want accurate info.