The function `parse-colon-path' changes the leading double slash
in a UNC path component to a single slash:

(let path (mapconcat path-separator "one" "//server/share/dir")
     (parse-colon-path path))
## -> ("one" "/server/share/dir")

A comment in `parse-colon-path' says:

;; Previous implementation used `substitute-in-file-name'
;; which collapse multiple "/" in front.  Do the same for
;; backward compatibility.

However, `substitute-in-file-name' does not do that:

(substitute-in-file-name "//foo/a/b") // -> "//foo/a/b"

There's no reason to do it in `parse-colon-path' either.