If gpg.conf contains an entry for local-user and gpg is called with the
--local-user switch (or: call gpg with more than one --local-user switch) and
the same keyid, then gpg errors out. IMHO it would be more consistent to work on
a use-last-specified base here, so the user can override the option from
gpg.conf via command line.

See Debian bug #130363.