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Time Lords can not cross their own time lines under normal circumstances. In "The Day of the Doctor" it was the Moment that made the Eleventh Doctor meet his earlier selves. On other occasions where the Doctor has crossed his own timeline(e. "The Three Doctors", "The Five Doctors", "The Two Doctors"), it was because some exterior force had made him cross his own timeline. As an example the Time Scoop from "The Five Doctors" was outlawed technology.