Two papers from different authors with identical content – which to reference?
Is this not considered a commitment of plagiarism?


If your account is accurate, yes. You should report your finding to both of the involved journals or publishers. Sites like Retraction Watch may also find this interesting, in particular if the journals do not react. (We have a few questions on how to best report plagiarism, e.g., this one.) You may also want to take a brief look at the publication history of the alleged plagiarisers and see whether you can identify similar incidents.


  Which paper shall I reference in my project paper?


If you can identify the original with some confidence, cite that. Why should you credit a plagiariser? Note that the original may not necessarily be the one with the earliest publication date (also look at submission dates, published preprints, or similar). If you are unsure, just cite both, and possibly add a remark that you noticed the anomaly and reported it (or similar). Though playing plagiarism detective may be an educative exercise, it is not your duty to do anything beyond reporting something fishy.