@whuber I've done it myself (answered something that might be seen as a duplicate) but generally either it's because I've forgotten that I've ever seen it before (in which case, definitely my mistake), or sometimes I'll do it when I do locate some near duplicates but there seems to be an aspect of the question that I don't think is addressed in the earlier question (and which isn't relevant in an answer to the question it might otherwise be a duplicate of).

Sometimes in such situations it can be hard to decide what to do.

I can't either remember or find with searches all the answers I've given, let alone 25 times as many on the site in total.

In some cases I've spent up to an hour looking for answers I am fairly sure I have given.

@HalilPazarlama I would always expect something described as "k-fold CV" to use randomly selected folds, unless specified otherwise.

@Glen_b I have had the very same experiences. I have even closed a few threads, where my answer was accepted, because later I discovered they were duplicates of other threads--that I had also answered!

I have been frequently grateful for community members who have identified hard-to-find duplicates. It's hard to be vocal about it: the natural course of action is to close the affected threads, and that's that--no rewards, no recognition, nothing. I just trust you all know who you are. :-)

@whuber "I trust you all know who you are" kinda perpetuates the "no recognition, no reward" notion. ;-p