Board Thread:The Flash discussions/@comment-32884340-20180601171052/@comment-33205590-20180601202255

Speed Force protects this from happening, I guess. However, there have been certain other instances in which non-speedsters interacted with themselves without creating anachronisms, such as Martin meeting his 1975 and 1987 selves and Damien Darhk meeting his 1962 self. So maybe the Legends broke time because it's a large group of people.