You may be extremely surprised by the answer. Because the answer is yes.
In 1953 a human chimera was reported in the British Medical Journal. A woman was found to have blood containing two different blood types. Apparently this resulted from cells from her twin brother living in her body More recently, a study found that such blood group chimerism is not rare. Another report of a human chimera was published in 1998, where a male human had some partially developed female organs due to chimerism. He was conceived by in-vitro fertilization. In 2002, Lydia Fairchild was denied public assistance when DNA evidence showed that she was not related to her children. After hearing of a human chimera in New England, Karen Keegan, it was eventually found that she too was a chimera and thus had two sets of DNA. Cyclist Tyler Hamilton, whose blood was found to have a “foreign blood population” in an anti-doping test, initially claimed it was natural, and his defense team suggested he could be a chimera. However, this defense was rejected.
Here is an excerpt from ABC’s story She’s Her Own Twin:
Lydia Fairchild was a proud mother who faced the most unusual of challenges. She had to fight in court to prove the children born from her body were her own.
“I knew that I carried them, and I knew that I delivered them. There was no doubt in my mind,” Fairchild said.
Fairchild’s fight for her kids began when she was 26-years-old, unemployed and applying for public assistance in Washington state. Everyone in her family had to be tested to prove they were all related.
The Department of Social Services called Fairchild and told her to come in immediately. What Fairchild thought was a routine meeting with a social worker turned into an interrogation. The proud mother was suddenly a criminal suspect.
What has also surprised me is that this phenomenon is not that rare:
Twin blood group chimerism seems to be very rare in humans. The 30-40 previously reported cases usually were found by mere coincidence during routine blood grouping in hospitals or blood banks. Usually in these cases frank blood group mixtures of, for example, 50/50%, 25/75%, or 5/95% at most were seen. Smaller percentages are very difficult to notice during routine work-up. Using a sensitive fluorescence technique (sensitivity > 0.01%) we detected blood group chimerism in 32/415 (8%) twin pairs and 12/57 (21%) triplet pairs, respectively, which is a higher incidence than reported previously.