What's the correct approach?
the correct approach is to read through threads before posting. If its one that has laid dormant for months or years, consider whether there's a reason to revive the thread. A warning will even come up if its been more than 6 (?) months since the last post. If several previous posters have already made similar suggestions, ask yourself whether your response adds anything to the discussion.
It's not unlike group conversations in real life. Generally it's rude to suggest the same exact thing that someone else has already said, and bringing up a topic that was already discussed extensively gets tedious for those involved and suggests the poster wasn't paying attention.
tl;dr - If you wouldn't do it IRL, don't do it on the forum.
I'm going to full on disagree with this, for the reasons listed by
@Omy and
@bisunacceptable above.
Also, online boards and threads are not real life conversations.
For one thing, they do not happen in real time anyway, even for those that happen over the course of only one day, so a latecomer cannot be compared to a RL conversation-buster.
They are also open to the public, generally, unlike private real life conversations. And suggesting the same thing as someone else is generally accepted when done as a "+1" or "ditto." Not to mention that adding to a prior suggestion can sometimes give it more weight, especially if it's an uncommon approach.
And, while the original poster may be long gone or have solved his/her issue, there may be 20 other lurkers who can benefit from the suggestions.
I do recognize that long-time posters can get weary of seeing the same topics over and over. That's not unique to this forum. It happens all over, and it's one reason that long-time posters tend to leave. Doesn't mean that new posters should miss out on being able to contribute to and extend an existing thread.