Let me explain the question with an example.
Let us say, I create a new branch from my
git merge origin/mybranch
git merge mybranch
A branch in git is just a human-friendly symbolic name of a commit. So, if branches point to the same commit - the result of merge will be identical. You could also could do
git merge <commit_id>.
The only noticeable difference is a default message text of the merge commit if any (and you could edit it if you want).