Paper in high impact journal duplicates work done a decade prior, how should I respond?
This is a very opinion-based answer, but ...


  What is the protocol for this situation?


Letting it go. 

You say that the paper got published in a high-impact journal 4 years ago. It reproduces results from another well-known paper. It is pretty much guaranteed that you are not the first person to notice that. It seems unlikely to me that the paper is indeed perceived as "suspect and unethical" by your peers.

Maybe your peers value the "more concise" write-up? Maybe there is a subtle difference that you have not noticed? Maybe, for some types of (empirical) papers, having a reproduction paper that basically just confirms that the observations from the first paper are indeed general is a really valuable contribution (although not citing the first paper is of course very bad in this case).

If you feel the case is indeed bad enough that something needs to be done, you have two options:


Mail the authors of the first paper.
Mail the EIC of the journal.


However, realistically, the chance of retracting the paper based on a years-delayed decision that the contribution was not novel seems very small, as long as there was no clear case of unethical behavior or a significant technical problem with the results.