Add the .mailmap file

Add the .mailmap file

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