The myth often implies that the cloned chip can then be altered with a different photo or personal data (eg the "Elvis" e-passport story). While once the data is altered this is no longer a cloned chip (ie an exact copy), any tampering will be detected by the digital signatures and the PKI authentication process.
An optional specification in the International Civil Aviation Organization Doc 9303 standard is "Active Authentication" (AA). AA works by having a private/public key pair, where the private key is imbedded in the chip and cannot be read out. If the public key is then copied (cloned) to another chip along with the rest of the data, the keys will no longer match and an AA authentication check will reveal this. Many countries have adopted AA and this will effectively eliminate cloning, although we believe that cloning was never a serious vulnerability.
Courtesy the Keesing Journal, 2009