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The
-f(force flag) is recommended in order to avoid conflicts with other developers’ pushes. Since you are not using Git for your revision control, but as a transport only, using the force flag is a reasonable practice.
Note: if heroku ever asks you to login trying to do a git push, it maybe more convenient to ctrl-c out, and then login with heroku login in the browser instead. This auth magically applies to subsequent command line and git push too.