mailmap: remove duplicate emails and normalize mapping entries

Change entries of the form:
"Proper Name <proper@email.xx> <proper@email.xx>"
to:
"Proper Name <proper@email.xx>"

This makes it clearer that no e-mail address mapping takes place in these
entries, only the "Proper Name" is mapped based on "<proper@email.xx>".

Normalize all e-mail mapping entries to:
"Proper Name <proper@email.xx> <other@email.xx>"

This makes it clearer which e-mail address is being mapped and follows the
formats listed in the gitmailmap documentation.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>