There seems to be a lot of disagreement about what constitutes deja vu. The author of that second article, for example, contends that deja vu only
involves the sense of sight, and that no one claiming to have had more than two deja vu experiences in a lifetime should be believed!
Well, that's just nonsense. I've had at least a hundred deja vu experiences, and they've all involved most or all of the senses. For me, and for everyone else I've ever talked to about the phenomenon, it's not simply finding a place familiar even though you've never been there -- it's feeling that you are literally reliving
some specific moment that happened in the past.
For me, it's never anything significant: just a ten- or fifteen-second snippet of specific conversation in combination with a particular action, a detailed visual image, sometimes even a smell or a taste, all occurring together in a way that I very clearly
remember as having already happened at some point in the past.
What I've always heard as the leading theory is that your brain simply slips (perhaps in a tiny epileptic seizure, as suggested in that first article) and processes some new piece of experience as if it were a memory instead. That seems logical to me, though I've never heard the actual mechanism by which it happens explained.
During the last few months of my father's life, as both Alzheimer's and the many medicines he was taking affected his thinking, I noticed that he had many more deja vus than usual... and longer ones, too. This may support the idea that it's simply a "mistake" in the brain's functioning.
Precognition and clairvoyance are totally different experiences from deja vu, but also legitimate, I believe. No idea how to explain 'em, though!