Probably the 'no duplicate link' exception doesn't apply to sites like tumblr where different subdomains likely belong to different and unrelated people.
ie:
hello.facebook.com and sayhi.facebook.com belongs to the same entity but you can't say the same for
ie:
hello.facebook.com and sayhi.facebook.com belongs to the same entity but you can't say the same for
hello.tumblr.com and sayhi.tumblr.com
I'm jx making a guess though