A woman with a rare birthmark has learned that she is her own twin.

Singer Taylor Muhl, from California, has a rare birthmark down the center of her body. One side of her torso and back appears slightly redder in color. She also has several other traits that indicated something unusual.

"Everything on the left side of my body is slightly larger than the right side," she writes on her blog. "I have a double tooth in the left side of my mouth and many sensitivities and allergies to foods, medications, supplements, jewelry, and insect bites."

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After talking to a doctor familiar with rare medical conditions, she discovered she has two sets of DNA.

"Not only that, I’ve been told I have two immune systems and two bloodstreams."

Taylor has tetragametic chimerism. This happens when two separate eggs are fertilized by two different sperms, and then one of the embryos absorbs the other during early stages of development. People with this type of chimerism may have two blood-types, as Taylor does, or different eye colors.

Chimerism is rare, with only around 100 documented cases in humans. People with the condition can live with no complications from it, other than altered pigmentation. Several people have only become aware of their condition after blood tests. Back in 2006, a mother had to prove her children were her own after a blood test showed that her DNA did not match the DNA of her own children, due to the presence of the DNA of her twin.

Taylor told her story to American talk show The Doctors.

In chimeric patients, the majority of cells usually end up coming from one set of DNA, Live Science reports. In some cases, the person can develop ambiguous genitalia, if the twin embryos they are developed from contain different chromosomes (ie if one twin is male and the other is female).

In Muhl's case, both embryos she developed from were female. However, she has suffered health problems because of the mix of DNA.

"My body reacts to my twin's DNA and cells as foreign matter, compromising my immune system to be much lower than an average person’s. Due to that, I, unfortunately, suffer from autoimmune and health challenges."

Taylor first discovered that she was effectively her own twin in 2009, and went public with it late last year to spread awareness of the condition and its link to autoimmune disorders.

"I felt it was important to show my entire physical demarcation because in the entertainment business there's an overwhelming amount of pressure to appear physically perfect or within a certain physical stereotype," she wrote at the time.

"I hope by showing my own imperfections I can inspire others who have physical abnormalities or who are different, to never let that stop them from pursuing their dreams, to love themselves unconditionally and to know they're beautiful just the way they are."