I'm not sure what the best way to check for duplicated vertices is. But I just selected all of them and welded them, moving up the threshold, and I didn't find any.
I know that this is already solved, but I want to point out that I noticed that in the normalmap texture, the problematic part seems to be twisted around. Notice that the problematic bits are diagonal. The very topmost vertices must have been grabbed and turned around slighty. This "seams" to be the culprit.