Ignore duplicate LDAP server config entries

This is a workaround for the problem that entries read from a global
GnuPG config file are written to the user's config file so that on the
next reading all entries are duplicated. This is an inherent problem of
the gpgconf API for list config entries. The best we can do is to hide
the duplicates from the users. To avoid the problem in the first place
the entries in the global config should be marked as immutable.

• GnuPG-bug-id: T7828