I think this happens when someone sees a new question and thinks he has a good answer for it, takes some time to craft a reply, and by the time he actually posts it, someone else has gotten in with a very similar answer. A kind of posting lag. When he discovers this, maybe he feels compelled to explain why he posted redundantly, perhaps to avoid a downvote.

Update: Looks like someone already said this.

Additionally, it can be seen as a compliment that they did beat them to it.

Elda Taluta; Sarks Sark; Ark Arks

Posting lag is the main issue for me, combined with the lack of an ability to casually delete my posts. It's embarrassment, basically.

Sometimes I try to add something with my "d'oh" update, so it doesn't just become "Solution. Update: jdporter already said Solution. I am a slow-witted idiot."

It reassures the OP that the solutions are correct~

Also, I try to host weekly trivia/riddle games in the CB. :P

I remember countless "How do we make the Super Search page more noticeable to new users" (esp. in CB) so people don't have to waste time on the countless RTFM questions. It's unavoidable though, might as well give up and make a browser shortcut for pasting "RTFM, please RTFM".

I don't mean the "If you'd searched the site, you'd have seen someone asked this question before!" comments. I mean the "Someone responded to this question before I did!" updates to answers.

Oh, that's definitely different :).

Either people looking for votes or not wanting to look like they were copying then, personally I don't care if someone posted before me. Though if they do I try to add an update to my post to an alternate solution or source of information, just to be different.