Error: Permission to user/repo denied to user/other-repo

Just remember to ssh-add all your shh keys for all your github accounts.

ssh-add ~/.ssh/id_user2
ssh-add ~/.ssh/id_user

You can also try to fix things by

git remote rm origin
git remote add origin git@github.com-user:user/repo.git
git push --set-upstream origin master
git config user.name "user"
git config user.email "user@domain"
git add .
git commit -m""
git push

And remember to add host based configuration into “~/.ssh/configuration”

Host github.com-user
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_usuer

Host github.com-user2
    HostName github.com
    User git
    IdentityFile