Operation Double

Operation Double was a plan organised by Osgood and her Zygon double, (TV: The Zygon Invasion) a result of a confrontation between the Unified Intelligence Taskforce and an incursion of Zygons who had came to Earth centuries prior after their home planet was destroyed in the first days of the Last Great Time War, with the War, Tenth and Eleventh Doctors, forcing both parties to negotiate a ceasefire. (TV: The Day of the Doctor) It was this plan, partially formed by Zygon High Command, that would allow every Zygon on Earth to live peacefully alongside their human counterparts. (TV: The Zygon Invasion)

Following Operation Double, Zygons were protected on Earth. Other shapeshifters, like Ballon, posed as Zygons in order to hide on Earth and avoid detection by UNIT, all while enjoying their protections. (TV: The Metaphysical Engine, or What Quill Did)

Mayor Me considered her hidden trap street, which had existed in London for at least a century prior to Operation Double, to be a more favourable way of housing aliens on Earth as opposed to the settling of the Zygons amongst humanity. (TV: Face the Raven)