Is re-discovery plagiarism?
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.