Plateflaw,
Great question!!!
My guess is it will differ depending on who you ask. I recently discovered a stamp with a double overprint hiding in a collection I had purchased on
ebay. I had already removed the items I originally bought the album for, and was going back though the album several months later, and something looked odd about this stamp. When I looked closer, I couldn't believe my eyes! Scott lists R191 with multiple overprints, but not R185. I have the discovery copy - right?

Plain as day - two overprints.
So, I did what this forum recommends daily - I submitted it to the Philatelic Foundation for a cert.
Imagine my surprise when the envelope came back and I opened it up!!!

Huh?
I did a search to see what the R191s looked like. Not exactly dark and complete strikes. Here is an example of R191 from PF Cert 398860

Murmur!!!
On the plus side, PF estimated the value of my discovery at $1, so I have that going for me!!!