If answers to the earlier question could be copied to the new one directly and would provide the answer, then definitely flag as a duplicate.
I generally also flag as duplicate if a good answer to the old question should answer the new one, even if it doesn't. (Rationale: it's better to improve things in one place rather than scatter things around. Occasionally, if the new question is better phrased and has already attracted better answers than the old one, I mark the duplicate the other way around.)
The third case is when the question is a very specific one about, say, camera model C vs. camera model N — in that case, we have a series of general canonical questions which are appropriate duplicate targets, even if they don't get to the precise "tell me which one is better" question (which we shouldn't be answering anyway).
Finally, sometimes the question shows a basic misconception or comes at something from a unique angle, where the knowledge in the other question and answers would logically address the issue, even if it doesn't serve as a direct answer. In that case, I sometimes do mark it as a duplicate (but wouldn't if I had gold-badge or moderator instant-close), but it's probably best to provide an answer explaining the issue/confusion and link to the other answer.
In the VR question you give as an example, you did the right thing.
