Cloning the data won't help. The data on any half decent phone is encrypted. The encryption key is kept insecure a secure memory, which requires the password to unlock.
When you use the duress password, it doesn't overwrite the data on the flash memory, instead that key is wiped. The secure memory has an integrated processor that generates a new key.
Merely having a copy of the data is useless, because unless there is a flaw in the key generation there is no way you can decrypt it. If there is a flaw, a mere border agent probably isn't going to have access to that information, it's the sort of thing that the NSA keeps to itself.