Add de-duplication of subkeys on a key

GnuPG will incorrectly add a pre-existing subkey that it doesn't
understand (e.g. ECC) to a key. This results in keys with a large
number of identical subkeys. Avoid this by detecting such keys and
de-duplicating the identical subkeys.

(gnupg bug report at https://bugs.gnupg.org/gnupg/issue1962)