I'm proposing a custom close reason specific to Puzzling Stack Exchange, that will sound something like:
This question appears to be off-topic because it already exists elsewhere in an identical form, in an easily-accessible way, and already has known helpful solutions. For more info, see [insert helpful links].
My reasoning for this proposal is many-fold, but I'll try to keep this short and to the point. It addresses many of the problems we've encountered all at once. But first, a bit of philosophy:
Questions that exist elsewhere, if they're easily accessible, do not grow or improve the art of puzzle-solving when copied here. Stack Exchange is designed to be a home for expert answers to expert questions. We're not just here to gain internet points for are awesome puzzling skills (or Google-fu, as the case may be). We're here to create a resource for people who are looking to further the art of puzzling as a whole, in any way, shape, or form.
While these types of questions aren't contraindicated for our success as a question-and-answer site, they certainly aren't constructive toward our ultimate goal, either. Keeping them isn't awful. Redirecting our efforts may be better.
On a more local level:
This is a step in addressing the chestnut problem. What's a chestnut, you ask? It's just one of those puzzles you hear from time to time if you keep your ears open long enough.
While I highly recommend reading the chestnuts post, it boils down to a couple things. We're likely to stumble across these puzzles anyway over the course of stumbling around the internet/books. We're not here to collect content. We're here to improve on it, and build it up. We're here to advance the art of lateral thinking, and copied content doesn't do that. It just stores it.
Many of these questions are outright plagiarized/dealing with attribution is a pain. Stack Exchange as a whole has a plagiarism policy, which we recently clarified due to some discovered abuse. This is certainly not the most important reason on this list, but it's a factor. If we just close copied questions, checking for attribution becomes less of an issue since the content wouldn't be acceptable here anymore anyway.
They're just all-around low quality questions. If you take a look through our most upvoted questions, very, very few of them are questions that can be found easily elsewhere. Sure, some of them might be linked to other sources or ideas, but most of them are not possible to find in their form as written. In other words: our best content is original.
I've gone on quite long enough. tl;dr: Copied questions don't improve the art of problem solving. We're not here to collect content that can already be easily found. We're not here to store old information. We're here to advance lateral-thinking and the state of puzzles.
Before any change like this can be implemented, community feedback and approval is needed!
So, what are your thoughts, community? Good thing or bad thing?