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There is no mechanical reason they could not be turned on a lathe, but it is my experience that the cost of turning is more than it is worth when you can get new rotors for a relatively small amount more - at least if you buy the less expensive rotors that I have found here on the forum.
Just had my rotors turned and they came out great.
GM Dura stop cross drilled and slotted rotors. The shop will inspect and measure the rotos and verify not defects and meet specs.