This article is from the All My Children FAQ, by Margaret Gibbs gibbsm@no_spam.poppyware.com with numerous contributions by others.
2 sets of twins, 2 sets of sisters, and 3 sets of unrelated
Kitty/Kelly (twins, Francesca James)
Adam/Stuart (twins, David Canary)
Cindy/Karen (sisters, Ellen Wheeler)
Natalie/Janet (sisters, Kate Collins)
Tad/Ted (unrelated, Michael Knight)
Jenny/Sheila (unrelated, Kim Delaney & Cynthia Sullivan)
Will/Justin (unrelated, James Patrick Stuart & James Garde).
James Patrick Stuart shared "Justin Carrier" duty with James Garde;
whenever the character Justin donned his "Will" make-up, J.P. Stuart
would portray the role.