When using the basic log command one of the authors shows up twice:

zsh» git log --all --format='%an <%ae>' | sort -u -k2
Vincent Demeester <vincent@demeester.fr>
Richard Hartmann <richih+github.com@richih.org>
Richard Hartmann <richih.mailinglist@gmail.com>
Dieter Plaetinck <dieter@plaetinck.be>
Corey Quinn <corey@sequestered.net>
Gernot Schulz <post@gernot-schulz.com>

If you instead use the git shortlog version with the .mailmap file you get:

zsh» git shortlog -se --all | cut -f1 --complement | sort -u -k2

Vincent Demeester <vincent@demeester.fr>
Richard Hartmann <richih.mailinglist@gmail.com>
Dieter Plaetinck <dieter@plaetinck.be>
Corey Quinn <corey@sequestered.net>
Gernot Schulz <post@gernot-schulz.com>

Alphabetical sorting on the second field (which, incidentally, may be a middle
name) is preserved.