On this site, people are incredibly liberal with voting: almost all answers that are remotely related to the question are upvoted, and any downvoting is called into question as being rude, mean, or uncouth. If you want to follow etiquette, you should upvote all the answers.
However, your own personal preference in how you vote isn't going to make or break a person's reputation total. So feel free to ignore the etiquette and follow the overarching principle of upvote answers you think are useful, downvote answers you think are not useful. Of course, only you can tell what's useful to you. If answers duplicating already perfectly suitable answers are useless to you, downvote without pity, or remorse, or fear and upvote only those answers you personally found of use.
To give you my own criteria in the situation you're describing:
I know that a lot of times, even so-called "subjective" questions really only have one real answer that people understand in slightly different ways. It's possible that people act in bad faith when answering (hoping you won't see they just duplicated an earlier answer for quick rep points), but I think most people are thinking that their answer is truly novel because it's not an exact copy-and-paste of an earlier answer.
Because of that, I tend to upvote the answers in which people explain their positions well. This dovetails nicely into the Six Subjective Guidelines which encourage questions to invite long answers that share reasons, experiences, and facts. It's not simply enough that one answers a question: we get enough answers already. For an answer to be useful here, it needs to provide more.
So if a person says something first but provides no detail, and a second person provides the same answer cogently (that is, they provided useful reasons why one ought to believe the answer is true), I'll always upvote the second person and I'll tend to downvote the first person.
If, however, the first person gave a really useful answer and the second person didn't provide any new insights to the answer, I'll upvote the first and downvote the second as the second isn't useful.
If both people have the same answer, but provide really great and different reasons for arriving at that answer, I'll upvote them both. Finally, if both answers lack any explanation, I'll downvote them both.