I wouldn't count re-reads. I used to read The Sun Also Rises every year and I read Desert Solitaire when I travel to the Southwest (yearly). But I also don't count the number of books I have read/

I'd say it depends on the wording. "Read 100 books". Any 100 books, nothing said about what kind? I can see it counting. But if the intent was to expose you to new things, I'd say no. Grated, rereading a book might allow you some insight on something you missed before, but you know the end of the story already. There is certainly gray area here, so in this, wording matters.

Thrice is Nice daddy wrong you right

Rereading a book definitely counts toward your reading goal.

Re-reading is a way of gleaning even more from a book than was had in the first or even subsequent readings of the text. I re-read all the time.

If you’re spending time reading it, yes, it counts.

I would say, it depends on your goal. Was it to read 100 books this year? or is this your lifetime tally?
I think, you can’t count the same book twice under, say, this year’s goal. But having read a book long ago shouldn’t count against you.

of course it counts

It absolutely counts. Besides rereading in order to reacquaint yourself with a series before you pick up there next book, rereading just because you enjoyed a book should count. Furthermore, saying it doesn't ignores the possibility of learning new things after rereading. Some books only grow richer the more often you revisit them. There are books which I'll read many times over the course of my life and continue to gain from.

It counts primarily if you are at a different stage in life and read it differently. I read novels that I encountered 20 years ago and it is a distinct experience, enriched by my examined life in that same time.

Absolutely!

Yes. They count for your short term goal.

Rereading counts.

Selective amnesia? Book passages that put you to sleep?