Yea its more likely to get a second A when you have a first A.
Anyway my source was the department of education website. Cant be bothered to find the link now. However, you have to take into account that any A-level is an a-level as far as they are concerned, and so they also include subjects like media studies and general studies, which are total mickey mouse subjects. Thus, from this I think we can conclude that those people getting AAA or better who have only respected science subjects are much rarer than 1 in 10. Infact, I only know 2 other people in my school who got AAA and their subjects werent total ****e.