A few weeks ago, I had a dream that included, among others, the man who hired me for my first engineering job. He died at 62, 2 years after he retired. 
Last time I saw him was over 10 years ago. He was a really nice guy and one of the 2 best bosses I ever had. Not sure why I dreamed about him, tho.
Funny enough, I had a very similar dream recently. I, too, in the back of my mind knew my dad was supposed to be dead - yet here he was talking to me like nothing happened. Unlike you, I instinctively knew that asking “Um, aren’t you dead?” would be highly impolite 
Really, that was my dream thought - “You can’t just ask him if he’s dead, that would be rude.” No idea what we talked about, but it seemed real at the time.
It can work both ways, though. One of my most realistic dreams I ever had was five or ten years ago when I dreamed that my uncle (who’s been like a father to me) had died. I didn’t see the body in the dream, nor did I actually dream of being told per se: In the dream, I was just walking down the sidewalk, at a spot near my my mom’s house, being sad about his death. Nothing odd about it at all, and some day in the future I presumably shall be walking in that exact spot mourning him. It was tough to go back to sleep after that one, let me tell you.
In waking life, my uncle is still alive and healthy (well, as healthy as any 70-year-old is, anyway), nor was he going through any particular health problems when I had the dream.
Only once. My paternal grandmother was a minor character in a very odd dream that included a lot of other people. We didn’t speak though. This was over thirty years ago.
Many times. Most were more like visitations rather than dreams, as defined above.
Once, a relative who had committed suicide told me in great detail what dying felt like, but stressed that those details were specific only to her, as dying feels different for different people. She came back the next night, and i thought “O shit, is x going to haunt me now?” X “heard” my thought, was taken aback and felt like she had done something wrong, and vanished.
Once an aunt reminded me to send a photo to her daughter, and once a man I didn’t know well suggested I get in touch with his widow, an old college friend. I did, and a close friendship resulted.
The Long Island Medium could tell you if it was a dream or a real visit. I don’t dream much but had a visit from a friends grandmother I knew was her. A special relative gets messages from the other side and the receivers are greatful to get them here.
My best friend died in 01. We still get together a few times a year while I sleep.
I apparently have unresolved issues with my brother. He died in 1977, long story short because of booze and drugs. Our next-to-last encounter with each other at Christmas 1975; he was wasted, went off his rocker and tried to beat up dad, who was not in the best of physical health at the time. I intervened, wound up ramming his head into the refrigerator and then dragging his semi-conscious ass out of the house.
The following year when I came down to visit for Christmas, he was there at my parents’ house and left shortly after I arrived. I never saw him again; he died in July 1977. I occasionally dream about him, and every time I do, I am furious with him for being so fucking stupid as to waste his life the way he did. I’m mad at him right now as I type this.
Dad in 09. Mom in 10. Cousin last year. Still see them a couple times each. It’s kinda nice.
I regularly have a variant of a dream where it’s not clear if my mom is dead or not. It’s because she died four years ago Friday but I still can’t believe it. So in the dream I suspect she’s dead but I don’t know for sure and I think, well, I would have gotten that Phone Call if she did die. But only my father is around and he’s not telling me either way so I have to think she must be dead but maybe she isn’t.
I get them, too: mainly about my father, gone seven years now, or my great-aunt, gone over twenty years. Father is always accompanied by whichever family car we had when he would be the age I would be when I dream it. We’re usually on vacation or doing something pleasant. My aunt appears in dreams when I’m under stress at work.