Yes, when we cite something that was already cited by another author, we have to reference to both of them... But what would you do in the following two situations?

1) A direct quote from Paper-B was cited in Paper-A, so you've found and read the original of Paper-B. But then realised there are more appropriate gems in that paper than what that quote was about, and as a result you used a different idea from that paper. When referencing Paper-B, would you still cite Paper-A with it? (You are already citing Paper-A for something else anyway)?

2) At some point during your research you've read Paper-X and cited. Much later you read a new article, Paper-Y. Then after a while, you re-read Paper-Y and noticed a footnote mentioning Paper-X. Not saying exactly the same thing as you, but context is similar. Now, would you amend the reference of Paper-Y when citing Paper-X, even though you weren't aware of Paper-Y at the time?