Duplicating a repository on github

When following instructions from: https://help.github.com/articles/duplicating-a-repository

You need to change:

git remote set-url --push origin https://github.com/exampleuser/mirrored

into

git remote set-url --push origin git@github.com:exampleuser/mirrored

Otherwise you'll be asked for your Username:

$ git push --mirror
Username for 'https://github.com':