How are you creating the "duplicate copy" and are you using same color space profile as the original file (i.e. ProPhoto RGB, Adobe RGB, sRGB). If the original is a raw file you need to use ProPhoto RGB color profile, which is the closest to LR's native color space (Melissa RGB).
In Compare view click on the left image and look at the Histogram. While watching the Histogram now click on the right image. Does the Histogram view change at all? If so then the two files are not the same!