Refuse duplicate --user arguments to pkexec

This usage is clearly errorneous, so we should tell the users they are
making a mistake.

Besides, this allows an attacker to cause a high number of heap
allocations with attacker-controlled sizes (
http://googleprojectzero.blogspot.cz/2014/08/the-poisoned-nul-byte-2014-edition.html
), making some exploits easier.

(To be clear, this is not a pkexec vulnerability, and we will not
refuse attacker-affected malloc() usage as a matter of policy; but this
commit is both user-friendly and adding some hardening.)

https://bugs.freedesktop.org/show_bug.cgi?id=83093