The thought of having an identical twin has always peaked my interest. To think that there would be someone that shares all of the same DNA as you is something that is almost unbelievable. This is why identical twins became the forefront of experimentation during World War II. When twins would get sent to the concentration camps they would be separated for the rest of the group. The group of twins would be treated better than everyone else and got privileges that the other residents did not get like eating everyday and playing sports. This was the good, the bad was when the experimenting started. They would sometimes infect one twin with a disease, like TB, and left to die and then the healthy twin would be killed so that they could compare them. Sometimes blood would be taken from one twin and transfused into another. They would also use painful eyedrops in an attempt to create blue eyes, these drops would cause infection and blindness. The though behind these experiments was that since the twins were identical in every way, they could see the changes that would occur when they were infected with a disease or see if there is a change when they were injected with the others blood. The reason that identical twins have the same DNA is because they are from the same fertilized egg. After the egg is fertilized it splits into two embryos that will be, basically, identical. The identical nature of the body parts of twins makes them perfect donors for each other. There was a story recently about a pair of twins that both underwent a rare breast surgery when they were diagnosed with breast cancer. One of the twins didn't have enough extra skin to be able to reconstruct the damaged breast so the other donated skin to help. This was also see back in the 1950's when one twin donated their kidney to the other. What makes twins so interesting is that many have no idea how they are conceived and how they can contain the same DNA, which is why they were the subjects to the testing back in war torn Germany. The unknown makes the science community go crazy and experiment so that the unknown can become the known.

References 

1. http://www.slideshare.net/guest54eeb4/nazi-experimentation-and-treatment-of-twins-1313734

2. http://www.nbcnews.com/health/identical-twins-share-breast-cancer-rare-surgery-2D11600484