Didn't this happen before? (Deja vu)

Not sure whether these thoughts are inspired by Neil Gaiman, Ted Chiang, or just recent happenings.

But recently, I've been having waves of deja vu.

Things like: I'm about to email someone at work for something, and I have this feeling that I've already done it. In fact, I can remember the chain of events leading up to it.

But there's nothing in my archives. And when I check with the person, yes, it's my first time asking for it.

Or, someone asks me for something, and I have the feeling this has happened before, and I know exactly what's going to happen next. And what my response is/will be. And a feeling that I have to respond in that way.

The feeling, like I've dreamt it before somewhere. And it's coming more often, and stronger, than ever before.

I wonder if people with photographic memory can remember their dreams.
I wonder, if dreams can portend the future, or possible futures. How can it be, when the future has not happened? Or if time is truly an illusion, and our paths are already set out from before we were born, or even before our grandparents or their grandparents were born?

And, if deja vu is happening more frequently, what does that mean? Is it like when you're near the side of the swimming pool, and the waves bouncing off the wall are coming back more quickly and higher?