Since the UID is not writable, it is not possible to create an NFC chip with a specific UID; meaning that it is not effectively possible to clone an NFC chip.
It is possible to create an NFC chip with a specific UID; this is done at time the NFC chip is manufactured and requires very expensive equipment. “Ordinary” companies, even GoToTags do not have the equipment needed to create an NFC chip. Additionally, manufacturers such as NXP also had features to their NFC chips to verify the chip was produced by NXP and not a counterfeit chip. This additional cost, complexity and additional security features results in an NFC chip being effectively un-clonable.