Is re-discovery plagiarism? I have a hard time understanding statements such as:


  Scientist X discovered Y in 1960 and was subsequently rediscovered by Scientist Z in 1980.


How does one prove that plagiarism has not taken place? This is also extensively observed in some really old math theorems, chemistry and so on. How does one prove that the similar work produced was as a result of one’s own independent work and not resulting from another’s. Even if something was re-discovered subsequently, why is it even given merit?
Remember that research was a lot harder to track down before the Internet Age. We couldn't as easily do literature searches because databases were much more restricted in scope and scale. Therefore, we can guess that keywords might not have found the earlier results.

In chemistry, it's possible that alternate methods could be used to synthesize a molecule, which would certainly justify its republication. In mathematics, an alternate, more elegant route for the proof or justification would also merit "rediscovery."

It's only plagiarism if the same methods and techniques were used after consulting the old material: in other words, only if it's a straight-up reproduction.