Can `git config` override entries in .gitconfig?

Is there a direct way to change the user.email entry for a git 
repository for one user (applied to all previous commits)?

I tried `git config --unset user.email` followed by `git config 
user.email email2` but it just sets a second field called user.email 
that shows up in `git config -l` as a duplicate. My ~/.gitconfig file 
contains email1 for the user.email entry.

Also, when the repository is created can I specify a second set of 
contact information (i.e. using a project specific email) which isn't a 
part of ~/.gitconfig?

output of `git config -l`:
user.email=email1
..
..
user.email=email2

Regards,
Nick