How to push existing source code to a repository in github?

I have a rails app hosted on heroku and i want to push the code to github(Aready created a repo on there).I tried to follow the steps in this question .Since i already have a local repository and currently can push and pull from heroku i skipped step 2 and 3 .But when i do step 4 git remote add origin git@github.com:sparkz19/stark-journey-1727.git It says fatal: remote origin already