Can you copy a tag whose UID matters?
Not onto a standard tag. The identifier is written at manufacture into memory that can never be changed afterwards, so any system that recognises a tag by its identifier will treat the copy as a different tag that happens to hold the same content.
Why the identifier cannot be copied
It is programmed at the factory into an area the chip serves read only. There is no command to change it, no tool that can, and no configuration that unlocks it.
This is deliberate. The identifier exists so a reader can single out one tag among several, and an identifier that could be set freely would be useless for that.
What breaks as a result
Anything that recognises a tag rather than reading it.
Phone routines triggered by a specific tag. Access systems checking an identifier against a list. Asset registers keyed on the identifier. Servers expecting a known value appended by mirroring.
In every case the copy holds the right content and is not the right tag. From the system's point of view a stranger arrived carrying a correct message, which is exactly the distinction identifiers exist to make.
Tags with a writable identifier
They exist and are sold openly, mainly for testing systems and for recovering deployments where an original has been lost.
Their existence is the reason identifier based access control is weak: if the value can be observed by anyone who reads the tag, and written to a purchasable tag, then knowing it is enough to reproduce it.
What they are not is a general solution. They are a specific product, they are not what a supplier ships when you order tags, and building a process that depends on them means depending on a niche item remaining available.
What to do instead
If you control the system, register the new identifier rather than trying to reproduce the old one. Adding a second trigger to a routine, or a second entry in a list, takes a minute and is the correct fix.
If you are designing the system, stop keying on the identifier for anything that matters. Use it as an index to look something up, never as proof that something is permitted.
And if authenticity genuinely matters, the answer is a chip that authenticates cryptographically. That is the only mechanism where a copy fails because it cannot prove what the original could, rather than because someone maintained a list.
Frequently asked questions
Can any tool write a new identifier?
Not to a standard tag, whatever the tool. Only tags manufactured with a writable identifier accept one, and that is a property of the product.
Is my access badge copyable then?
If it is checked by identifier alone, reproducing it is a matter of obtaining a tag that accepts a written identifier. That is precisely why identifier based checking is considered weak.
How do I add the copy to my phone routine?
Register the new tag as an additional trigger in the automation. The routine then responds to either tag, which is usually what you wanted anyway.
Does mirroring change this?
No. Mirroring injects the tag's own factory identifier, so a copy mirrors its own value rather than the original one. The server sees a different identifier.