You can't really prevent that unless you design a system which is inherently designed to track people, e.g. by phoning home to the issuer on each credential verification. The system being deployed right now is based on the issuer issuing batches of single-use credential tokens to device-bound single-use keys, which on the plus side means that colluding verifiers cannot use age credentials as cookies to track people. It is still vulnerable to colluding verifiers and issuers though, because the issuer can de-anonymize the tokens (it knows them and their linking to the identity of the user). This scheme also means that if the keys that the tokens are issued to are not device bound, then it is trivial to copy the age credentials to someone else.

To my knowledge, even more sophisticated ZKP schemes still rely on device bound keys to protect against duplication.