Millie, almost 16 now, and diagnosed with AS just a few months ago, has always had favourite clothes - one summer (not so very long ago, definately whilst in her teens) she spent all summer in a football (soccer) kit, complete with shin guards...
When she gets something new she wears it and wears it and wears it - it is very difficult to get it off her to wash it!
Worse than having favourite clothes though is her dislike of others - she hated frilly vests, or polo necks, or anything too tight, even from a very young age.