As pointed out by Jorg Kennis, Twitter‘s new lists feature make it hard to determine the amount of unique followers. I’ve written a simple script, using a slightly modified TweePy, to determine the amount of unique followers.
bas@w-nz ~/twitter-unique-followers $ python twitter-unique-followers.py JorgK -ubwesterb
rate_limit_status: remaining_hits: 112
followed directly by 182
RPtje/vriendenbekenden subscribed to by 0
JorgK/TechNL subscribed to by 0
sentfanwyaerda/nijmegen1 subscribed to by 9
JorgK/Friends subscribed to by 0
sjorsjes/Community subscribed to by 0
robinspeijer/iPhone subscribed to by 1
nielsschooneman/iPhone subscribed to by 0
JeanPaulH/iPhoneclub subscribed to by 5
number of unique followers: 198
You can download it here. I could write a simple webpage with the same functionality, if anyone would mind.