Github shows that my commits have been pushed by 2 authors: me and me

First, make sure this is authorship, not "author" and "committer".

If we are talking about two authors (as seen in the picture below) ,then this recent, since GitHub only announced it a few days ago (January 2018):
"Commit together with co-authors"

To add co-authors to a commit, just add one or more "Co-authored-by" trailers to the end of the commit message:

Commit message

Co-authored-by: Joel Califa <[email protected]>
Co-authored-by: Matt Clark <[email protected]>

Include your trailers at the end of your commit message, and have at least one line of white space before them.

See the official documentation.

In your case, that means any commit with multiple "Co-authored by" would be displayed that way:

How can I turn it off?

Make sure you don't have any Co-authored-by field in your commit message trailer.

If you don't, report that to GitHub support, as it is most likely a bug resulting from their recent feature described above.

its happens when your local Git configuration doesn't match with your Github account.

you can check this by:

git config --global user.email

to change your email configuration

git config --global user.email "your email"