Multi-part challenges aren't ideal, but shouldn't be forbidden

To be fair, "multi-part" is a relative term. A challenge that asks for one program that takes the square root of the cosine of a number is not all that different from a multi-part challenge that asks for one program that takes the cosine of a number and another program that takes the square root of a number.

Multi-part challenges aren't that common anyway (and don't seem to be getting any more or less common), so a few won't tarnish the site too much.

The primary complaints about them mentioned in the question were:

(1) Mainly it's not clear how we should deal with people copying one of the subchallenges from another answer, and (2) how these should be treated in the context of partial duplicates.

I'd argue that all challenges share issue (1) - copying sections of code or algorithm ideas can happen anywhere. It's easier in multi-part challenges but still, whether it's allowed or not is a different meta question.

Issue (2) is the real crux in my opinion, since we put so much effort into our avoidance of duplicates. I'd say that if no part of a multi-part question is a duplicate, why not allow it? The author still came up with the ideas themselves and why not let them combine them into one challenge if they want to?

If some parts are duplicates then that's a different story. I'm not really sure what the best solution is but here are a few ideas (maybe this could be its own meta question):

• Not allow any duplicate parts. (Encourage author to quickly change/remove offending parts.)
• Allow it if the majority of the parts are non-duplicates.
• Don't have a hard rule and let people vote to close as duplicate naturally. (Less official but perhaps more fair in some cases since it's kinda case by case.)

(Recent example of a multi-part question, just for some context.)