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I'd say if your entire app can be cloned in a day, it's fair game. Hardly rocket science.

What about issues of design? Even if the individual features are easy, it is hard to get the right mix and in the right spots.

As an analogy, is it OK to copy a font and call it your own?

Given the brief of "Make a webchat client", it's hard to come up with a different design that would work.

Like an email client, it has an obvious design that works.

Yes, there's no defensibility in functionality.