Duplicate data error after error in zone

I wanted to test the "occluded data below NS"-story. So I added some test data in a zone, but accidentally formatted the MX RR wrong. Fixed the error and resigned the zone. But then the SIgner Engine errors about duplicate data. To reproduce this error do the following:

• Start Signer Engine: *
  ods-signer start

• Add test data that is formatted wrong: *
  test.nz. IN NS ns1.test.nz.
  ns1.test.nz. IN A 1.2.3.4
  ns2.test.nz. IN MX mail.test.nz.

• Tell the Signer Engine to reload the zone: *
  ods-signer sign nz

• You get this error as expected: *
  Oct 14 07:13:00 fou ods-signerd: cmdhandler: zone nz scheduled for immediate re-sign
  Oct 14 07:13:00 fou ods-signerd: error parsing RR at line 31 (Syntax error, could not parse the RR's rdata): ns2.test.nz.^IIN^IMX^Imail.test.nz.
  Oct 14 07:13:00 fou ods-signerd: error reading RR at line 31 (Syntax error, could not parse the RR's rdata): ns2.test.nz.^IIN^IMX^Imail.test.nz.
  Oct 14 07:13:00 fou ods-signerd: error reading zone nz from file nz.inbound
  Oct 14 07:13:00 fou ods-signerd: task [read zone nz] failed

• Fix the error in the zone file: *
  ns2.test.nz. IN MX 10 mail.test.nz.

• Reload the fixed zone: *
  ods-signer sign nz

• But the you get errors about duplicate data: *
  Oct 14 07:13:16 fou ods-signerd: cmdhandler: zone nz scheduled for immediate re-sign
  Oct 14 07:13:16 fou ods-signerd: adding duplicate RR?
  Oct 14 07:13:16 fou ods-signerd: error adding RR to pending add RRset (6): duplicate
  Oct 14 07:13:16 fou ods-signerd: +rr nz.^I60^IIN^ISOA^Iloopback.dns.net.nz. soa.nzrs.net.nz. 2010070602 900 300 604800 60
  Oct 14 07:13:16 fou ods-signerd: adding duplicate RR?
  Oct 14 07:13:16 fou ods-signerd: error adding RR to pending add RRset (2): duplicate
  Oct 14 07:13:16 fou ods-signerd: +rr nz.^I86400^IIN^INS^Ins7.dns.net.nz.
  Oct 14 07:13:16 fou ods-signerd: adding duplicate RR?
  Oct 14 07:13:16 fou ods-signerd: error adding RR to pending add RRset (2): duplicate
  Oct 14 07:13:16 fou ods-signerd: +rr nz.^I86400^IIN^INS^Ins5.dns.net.nz.
  Oct 14 07:13:16 fou ods-signerd: adding duplicate RR?
  ....
  ....

Probably some leftovers from the previous error?