Proper citation of scooped or defective articles

If the paper duplicates or rediscovers earlier results, you can simply cite the earlier work together with the new one.

All splines are reticulated [Smith 1963; Jones 2008].

Or if only the earlier paper is really relevant, you can cite it alone. But if the Jones paper is commonly cited in this context, and you don't cite it, a reviewer may wonder why. You can of course explain why you don't want to, but it may cause more friction between you and the reviewer than you want. Or, your apparent unfamiliarity with the famous literature in your field may, in a borderline case, cause your paper to be rejected before you have a chance to explain.

If you feel strongly, you can say something like "The work of Smith is often overlooked, but is particularly significant because blah blah blah...". In other words, you can promote the work of Smith if you don't think it is as widely known as it deserves, but don't explicitly put down Jones.

If a paper has an error that is relevant to what you are discussing in your paper, then describe it in the text.

It was shown by Jones that every snark is a boojum [Jones 2008]; note that Equation 4.16 in that paper contains an error, where X should be replaced by Y.

I do not see any reason to mention that the authors have been notified. The reader will presume that you did so, because that's what any sensible academic would do. If the authors didn't do anything about it, you can take it up with the journal if you feel it's really important; but I think it's unprofessional to use your paper to passive-aggressively shame them.