Attached are the properties of duplicated edge of unrollsrf’ed polysurface.
-8.04538e-15 IS zero for all intents and purposes.
That’s -0.00000000000000804538
–Mitch
Technically it’s not. 
It was just 0 (zero) before the last update or a month ago. I don’t know exactly which update “broke” it.
Well, this has been argued before, as far back as Rhino 1.0 IIRC…
Rhino’s built-in "Zero Tolerance is 1.0e-12, this is 2 orders of magnitude smaller than that. It’s just floating point math fuzz.
However, I’m not seeing what you’re seeing. All the dupe edges of my unrolled surfaces are at ‘0’
–Mitch
BTW, what version of Rhino? Mac or Windows?
^ Exactly my point. Why don’t I see 0s?.. 
This is Windows version.
Dunno… I have the same version. (changed category to Windows)