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All I'm saying is that, when someone resizes and sharpens image (as it is usually done) dark or white edges appear, and that is the artifact. Dark or white halo. Different image requires different approach.. but sometimes you can just leave sharpening to zero, and resize 50% with bicubic, and you will get sharp image.
No sharpening done, only resize with 50% (two times =25%):
And this is arbitrary resize (73% for example..) with sharpening, (sharpen filter or any other) and then resized to same pixel dimensions as first image: