The polar opposite
While many people on this list have inherited their parents' unique DNA, others have the inverse. Consider this individual. We are convinced that when he goes shopping with his father, few people will notice that they are father and son. Apparently, the DNA issue caught this one off guard, and he grew up to be the polar opposite of his father.
Friendship
Birthmarks are one of our distinguishing features. Imagine their surprise when they learned they had practically identical birthmarks, but the color was radically different. The markings are on their arms, however one is darker than one of their skin tones, and the other is lighter. Their connection was definitely predestined.