The .mailmap feature is used to coalesce together commits by the same
person in the shortlog, where their name and/or email address was
spelled differently.
For further reference see the man page of git shortlog
git shortlog -sne |awk '{ print $NF }' |sort |uniq -d
git shortlog -sne |awk '{ NF--; $1=""; print }' |sort |uniq -d
--- /dev/null
+Miguel Ángel Arruga Vivas <rosen644835@gmail.com> Miguel Angel Arruga Vivas <rosen644835@gmail.com>
+Daiki Ueno <ueno@gnu.org> Daiki Ueno <ueno@unixuser.org>
+Guido Flohr <guido@imperia.net> Guido Flohr <guido@imperia.bg>