One person outside but two people inside. That’s the gist of the chimera, a human being who carries the DNA (and sometimes the body parts) for two. It sounds crazy, but it happens. In fact, doctors think it probably happens more often than we realize. Unless there were some reason to test the DNA from cells in different parts of your body, you could easily be a chimera and never know it.
A chimera is a single organism composed of genetically distinct cells. This can result in male and female organs, two different blood types, or subtle variations in form. Animal chimeras are produced by the merger of multiple fertilized eggs.

Tetragametic chimerism occurs through the fertilization of two separate ova by two sperm, followed by aggregation of the two at the blastocyst or zygote stages. This results in the development of an organism with intermingled cell lines. Put another way, the chimera is formed from the merging of two non-identical twins. As such, they can be male, female, or have mixed intersex characteristics.
This can produce an organism with totally different characteristics on each side of the body: different genders or different colouring — as in the budgie above. Or it can cause mozaicism where the organism contains patches of the different cell types, something most easily spotted in coat colouring — as in the human above — or eye colour.
But very often the mosaicism is there; we just don’t know it.