gpg: KEYTOCARD failed: Unusable secret key

[Felix E. Klee]

Successfully moved a key to an [OpenPGP-Card][1]. Now, as backup, I
want to install the key to a second card, but that failed:

    # gpg --edit-key $KEY
    [...]
    gpg> toggle
    [...]
    ggp> keytocard
    Really move the primary key? (y/N) y
    [...]
    Please select where to store the key:
       (1) Signature key
       (3) Authentication key
    Your selection? 1

    gpg: WARNING: such a key has already been stored on the card!

    Replace existing key? (y/N) y
    gpg: KEYTOCARD failed: Unusable secret key

Why did it work for the first card but not for the second one?

I assume, although `keytocard` is documented as *moving* the key to the
card, it actually copies it.

[1]: https://g10code.com/p-card.html