Paper in high impact journal duplicates work done a decade prior, how should I respond? This paper was published in a high impact journal for physics, Physical Review Letters, in 2010, and basically duplicates work done in 2000 by another author in the field, here.

What is strange to me is that, in our subfield of optical polarization theory and measurement, the author of the second, older paper, is well known.  There is no way that an adequate reviewer couldn't have known about the second author's work in 2010.  The 2010 paper doesn't cite this former work.

What is the protocol for this situation?

The first, newer paper, has a more elegant and easily understandable derivation, but it still duplicates the older paper, and isn't original, just more concise.  However, the 2010 paper, disingenuously states that "non-quantum entanglement" is used to resolve an issue (the issue solved in 2000).  This needs to be called out, because mathematically all we have is a 2 element vector with elements that are not linearly dependent, and this is what they are denoting the "non-quantum entanglement."

In my opinion, this lack of citation of the original work by the authors of the first paper above (newer), likely where they got the idea in the first place, is potentially unethical.

Edit:

Now that I've been in academia for awhile, I find myself often disappointed and disheartened by the following problems, most of which are primarily political : 


The unfortunate view that retractions are so unpleasant that they almost never occur.  Everyone is wrong occasionally, the literature shouldn't be polluted with incorrect results.
Everyone always being afraid to be wrong, and subsequently afraid to fail.  Failure is essential to developing new frontiers.
The lack of risk taking in mainstream science.
The focus on positive results, especially in experimental sciences.  I'm sure we would all love to know when not to spend years going down a fruitless path.
Ethical issues being addressed as just ignore it.


Retractions (for ethically sound reasons, like mistakes, etc.) really shouldn't be so unpalatable that we end up with incorrect results proliferating in the literature.

Ethical problems like the above, failing to cite likely known prior work shouldn't be swept under the rug and just ignored for political reasons.
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.