just git push after git checkout -b existingbranch

If we check out an existing branch, make modifications and want to push to that branch, we just have to do 

git push

and not 

git push -b existingbranch

or else we would get the error

existingbranch already exists

https://stackoverflow.com/questions/53046763/fatal-a-branch-named-new-branch-already-exists

https://www.geeksforgeeks.org/git/default-behaviour-of-git-push-without-a-branch-specified/