bug#34936: 27.0.50; duplicated process-environment items in pdumper

[Eli Zaretskii]

> From: Shuguang Sun <shuguang79@qq.com>
> Date: Thu, 21 Mar 2019 23:09:07 +0800
> Feedback-ID: esmtp:qq.com:bgforeign:bgforeign4
> 
> I start a personalized pdumper emacs, 
> emacs.exe --batch -l "~/.emacs.d/var/dumps/dump-init.el" -eval 
> "(dump-emacs-portable \"semacs.pdmp\")"
> (even with an empty dump-init.el)
> and there are duplicated items in process-environment:

I don't think re-dumping works well yet, but I see no reason not to
fix this particular issue.

Please try the latest master, I think I fixed this.