Some things known about déjà experiences:

Many have them (for example, a recent survey indicated the incidence to be around 63% of the adult American population).
There are multiple forms of "déjà" experience (Dr. Vernon Neppe, Seattle, has specified 4 main categories and - at last count - 27 types).
Déjà experiences tend to be banal
They seem to happen more often and be more intense when people are young (say between 15 and 25).

Many have them (for example, a recent survey indicated the incidence to be around 63% of the adult American population).