There's a bit of a continuum here:
- On one end we have answers that share a core idea, but are written very differently, possibly including different details, explanations, and so on. These are generally good: it gives us a chance to vote on the best way to present the information to readers.
- More commonly in the middle, we have answers that don't really add anything new, usually because they repeat the core answer and leave the details out. These don't have as much value to future readers. They're especially problematic when they're on questions that already have plenty of probably somewhat repetitive answers; more clutter makes things even worse for voters and for future readers.
- Far on the other end, we have copy-paste duplication. This is definitely not useful, but it's very rare.
With the middle category, it can be pretty tricky to decide which way to lean - is it effectively as useless as copy-paste duplication, or does it possibly provide better phrasing or a more concise summary that makes it actually useful? So as moderators we may or may not delete these unilaterally; we try to use our best judgment. (Users with enough reputation can vote to delete posts themselves as well, but I haven't seen this happen much on this site.)
Personally, I'm unlikely to delete if the answer seems well-written and there aren't too many other answers, and I'm more likely to delete if the answer is too short to be fully useful (possibly poorly written) or there are a ton of answers.
Given that, as a non-mod, you can still leave comments, and if you think deletion may be warranted, feel free to vote to delete and/or flag.
When I see late duplicate-ish answers from new users that I don't intend to delete, I generally write something like:
Welcome to Seasoned Advice! While this does answer the question, the existing answer(s) gave the same advice with more detail. I think you'll find your answers better received if you add new information!
and if I intend to delete, I might tweak slightly:
Welcome to Seasoned Advice! While this does answer the question, the existing answer(s) gave the same advice with more detail. Since there are already so many answers, in the interests of keeping the page organized and concise for future readers, I'm afraid I'll have to remove this post.
You could also sneak in variations on "thank you for your answer", and tweak it to say "it may be removed" if you're just flagging.
Sometimes it may be useful to point them to the tour/faq, but it's not too specifically focused on this issue, so I'd think of it more as a new user welcome thing. Sometimes it may also be useful to suggest that they suggest an edit to an existing answer (or to just do the edit for them).