Medical Deja Vu:
Research has revealed that in some cases, Deja Vu has been linked to temporal lobe epilepsy. It is believed to be caused by the irregular electrical discharges to the brain during epileptic seizures. These episodes often give the patient the sense that the present event has occurred before. However, there is no need to worry that you may have epilepsy because you experience Deja Vu. There are other theories about why it happens.
Deja Vu Related to Memories:
Some scientists have proposed that Deja Vu may be caused by an actual memory. An occurrence that is similar to what is currently happening may trigger the memory of a past event giving the person that feeling of Deja Vu. The event occurring in the present is most likely not identical to the past scenario but recognition memory has been triggered.
Dream-related Deja Vu:
This one is pretty self-explanatory. Sometimes, an event may be very similar to a person's dream. Again, the brain is triggered and immediately feels that these events have happened before leading to that familiar feeling of Deja Vu.
Paranormal Deja Vu:
There are those who believe that Deja Vu is linked to past lives that we have lived. There are many religions that believe in Reincarnation around the world. The major religions that hold this belief are Hinduism, Jainism, Buddhism, and Sikhism. Reincarnation is the belief that the soul can be reborn into another body and as such a person never truly dies. So a person over many lifetimes may have a repeat experience, resulting in Deja Vu. Of course, there is no evidence to support the belief of Reincarnation.
So it is clear, that Deja Vu still remains somewhat of a mystery. Many theories have been put forward, but none can be proven conclusively. So it may remain an enigma for future generations to solve