R15c Double Perf ?

This R15c looks to be doube perf'd horizontally. Luckily, it was separated from the stamp above & below via the outer perfs. Anyone seen this before?

Rows of extra perforations on first issue revenues are fairly common, and the general consensus seems to be that most of them are fake, added later in the 19th century when the perforating devices fell into private hands. However, I am inclined to think that double perfs on documents, such as yours, are legitimate, but I don't know how scarce such uses are. I have an R6c like that on the back of a photograph:

Quite common.

A recent acquisition:



I think these are probably legitimate. It's the crazy ones criss-crossing the stamp that tend to be suspect.

Thanks for the responses. I'm fairly new at collecting revenue stamps so it helps when I get informative responses. Appreciate it.