All parameters to 'git merge' in this case are branches that you're merging from, i.e. source branches. You're always merging to the current branch.
origin is a remote name, git automatically expands it to that remote's default branch, so it's actually equivalent to
origin/master – the command is being told to merge the same branch twice.
(It is possible to merge more than one branch, known as "octopus merge", but this is rarely done – and when it is done, the branches of course are different.)
As it is, the command doesn't make much sense. Maybe it should have been either
git merge origin/master (without the duplication) or
git pull origin master.