Doppelgänger is a German term that literally means "double-goer." It is used to describe a person's double or identical twin, who appears to be a physical duplicate of the original person. The concept of doppelgängers has been present in folklore and mythology for centuries and has been documented in various historical reports.

The earliest known reference to doppelgängers can be traced back to ancient Egypt. The Egyptians believed in a spirit called ka, which was believed to be an exact replica of a person's soul. This spirit was thought to leave the body at the time of death and continue to exist in the afterlife. The concept of ka can be seen in various Egyptian texts, including the Book of the Dead.

In Greek mythology, the concept of doppelgängers is associated with the idea of a person's double, which is said to be a supernatural being that looks exactly like the original person. The Greek poet, Homer, mentions the idea of doppelgängers in his epic poem, the Odyssey. In the story, the goddess Athena creates a doppelgänger of the hero, Odysseus, to help him regain his kingdom.

In Norse mythology, doppelgängers are called "vardøger." They are believed to be a premonition of a person's arrival, where their presence is seen or heard before the actual person arrives. In Scandinavian folklore, vardøger are believed to be the supernatural counterpart of a person, which follows them around and mimics their actions.

Doppelgängers also appears in literature. One of the most famous examples is Edgar Allan Poe's short story, "William Wilson." The story follows the life of a man who encounters his doppelgänger, who is an evil twin that tries to take over his life.

In the 19th century, there were several reports of doppelgängers. One of the most well-known cases is that of Emilie Sagée, a French teacher who worked at a girls' school in Latvia. According to reports, Sagée's doppelgänger would appear next to her in the classroom, mimicking her every move. The phenomenon was witnessed by several students and faculty members.

Another well-known case is that of Percy Bysshe Shelley, the famous English poet. Shelley reportedly saw his doppelgänger on several occasions before his death. The doppelgänger was said to be an omen of his impending death, which occurred a few days later.

Here are some examples of historical reports of doppelgängers:

1. Emilie Sagée - a French teacher who worked at a girls' school in Latvia in the 19th century. According to reports, Sagée's doppelgänger would appear next to her in the classroom, mimicking her every move. The phenomenon was witnessed by several students and faculty members.
2. Percy Bysshe Shelley - a famous English poet who reportedly saw his doppelgänger on several occasions before his death. The doppelgänger was said to be an omen of his impending death, which occurred a few days later.
3. Queen Elizabeth I - her doppelgänger was said to have been seen by many people, including her own courtiers. The doppelgänger was believed to be an omen of the queen's death and was seen shortly before she passed away.
4. Johann Wolfgang von Goethe - the German writer and philosopher claimed to have seen his doppelgänger while riding on the road to Drusenheim.
5. John Donne - the English poet and cleric reportedly saw his wife's doppelgänger in bed beside him while she was actually in another room.
6. Abraham Lincoln - the American president claimed to have seen his doppelgänger in a mirror, shortly before his assassination.
7. Guy de Maupassant - the French writer claimed to have encountered his own doppelgänger in the woods, which he believed drove him to madness.
8. Catherine the Great - the Russian empress was said to have seen her own doppelgänger sitting on her throne, which was believed to be an omen of her impending death.
9. Robert Dale Owen - the American social reformer and politician claimed to have seen his father's doppelgänger after his father had passed away.
10. Queen Marie Antoinette - the French queen was said to have seen her own doppelgänger while walking in the gardens of Versailles.
11. Sir Frederick Carne Rasch - the British politician claimed to have seen his own doppelgänger while visiting the Palace of Versailles in France.
12. Sir Arthur Conan Doyle - the author of the Sherlock Holmes stories claimed to have seen his wife's doppelgänger while she was still alive.
13. Auguste Kekulé - the German chemist claimed to have seen a snake-like doppelgänger that helped him to discover the structure of the benzene molecule.
14. Queen Victoria - the British monarch was said to have seen her own doppelgänger several times throughout her life.
15. Sir William Crookes - the British scientist claimed to have seen his own doppelgänger while conducting experiments in his laboratory.

These are just a few examples of reported doppelgängers. The phenomenon has been documented in many different cultures and time periods.

In modern times, doppelgängers continue to be a popular theme in movies, television shows, and books. The concept of doppelgängers has also been studied by psychologists and parapsychologists, who are interested in understanding the nature of the phenomenon.

The concept of doppelgängers has been present in folklore and mythology for centuries. Historical reports of doppelgängers suggest that the phenomenon has been witnessed by many people throughout history. While the existence of doppelgängers remains a topic of debate, the idea of a person's double continues to captivate people all over the world.