Invisible Gold in Asia Facebook Page (set up by author of the book with the same name Professor David Llewelyn) explains that both parties (or theoretically even more parties) can come up with the same copyrighted work: "Although that will be harder to prove so make sure you keep records."
If you can prove that you created the work independently, you can use it as a valid defense against copyright infringement.
See Jonathan Mak's design here.
The design of UK designer known as Raid71 here.
In this YouTube video the voice-over says that Jonathan Mak "has been offered copyright offers". Well, if Mak is the creator he automatically has the copyright of the work, at the moment of creation. What the voice-over probably means is that he got offers to assign his copyright.