services_mkdb and "duplicate service" messages

After adding a couple of entries to /etc/services I ran services_mkdb(8) to generate /var/db/services.db as instructed in the man page. services_mkdb(8) generates a huge amount of "duplicate service" messages. Is this normal and expected? I can't see any duplicate services in /etc/services unless the db generation ignores the protocol. My Berkeley DB knowledge is limited so I'm unable to compare the contents of the db file with the original services file. Please advise.