Board Thread:Arrow discussions/@comment-28109177-20171231211528/@comment-27700324-20180101195631

It is pretty simple. A time remnant of Barry was created in the future to stop Savitar. (Time remnants are created by going two seconds in the past so you can essentially make clones of yourself) This remnant survived and had been rejected by Team Flash for not being the real Flash. He goes back in time to turn past Barry into him.