Why not to stop upgradepkg from double-installing the package
since I was thinking about patching it out for speed on armedslack:

19:06 < volkerdi> 
If you patch it out, you'll realize the issue within a year or so.
imagine symlinks turning into directories and vice-versa
A good example is when the /usr/lib/zoneinfo dir moved to
/usr/share/zoneinfo.  There was a link from /usr/share/zoneinfo to
/usr/lib/zoneinfo, and in an upgrade the locations of the dir and
link swapped.  Pass one (ip), package is extracted through the old
link to the old dir.  Pass two (rp), rm -rf is applied to the old
link.  At this point there's no zoneinfo database.  Pass three (ip), fixes that problem.

Regarding pre-installing a package in upgradepkg:
The directory - symlink rationale for the install/uninstall/reinstall cycle is a rare thing.  
At this point, the .new files are the real reason.  
The problem happens when a package used to install /etc/some/conffile.conf directly, and then a newer package changes to using /etc/some/conffile.conf.new.