Doug is usually upfront when he's doing a song that he's heard before. I know he does do a decent amount of prep ahead of time (or "reading in" as he calls it) with regards to the song/album/band before, and some bands he's done multiple times can be somewhat predictable (looking at you Iron Maiden).
I actually joined Doug's patreon back when he announced that he was going to do all of SFAM and it's a pretty great community. I also ended up going to a Tool concert with him because he had an extra ticket to sell, nice guy in person.