What does "addnode: NSEC node already exists" mean?

[Alexander Gall]

I have a signed zone for which dnssec-signzone and named-checkzone of
BIND 9.8.0-P4 emit the message in the subject several times.  This
appears to happen in loadnode() defined in lib/dns/rbtdb.c and has
something to do with an "auxiliary tree for NSEC", whatever exactly
that is.  It doesn't tell me which NSEC record it's complaining about,
though.

Any suggestions by the ISC folks what this is about before I dig into
it deeper?

-- 
Alex