I know there are sites like copyscape, dupecop, etc, but what I'm after is a bulk article checker for plagiarism. You could throw a zip file with a bunch of articles at it, and it will list what articles are duplicated, and what are original. Does anybody know of such a tool?
That kind of software exists, but since tools like these come and go rather quickly in the blackhat Internet marketing/SEO scene (usually bundled with or integrated into other programs), please excuse my not being able to recall any names/links. As usual in that niche, most of the software is either outright junk or ridiculously overpriced, but if you are willing to take it upon you to dig through a bunch of all those "sign up for your free ..." pages, you will stumble upon spamming tools that include the functionality you desire.
I've created my own "bot" with imacros and http://www.plagiarismchecker.com/ Works pretty well. You must look into iMacros, some pretty powerful and easy stuff there.