Same zone records in bulk import are ignored

[ksyz]

When importing zone data in BIND format, and zone contains multiple records for the same label (even in case with A/AAAA types) only the last one is stored (with import_overwrite=on):

ns1.asf.as.              300     IN      A       127.0.0.1
ns1.asf.as.              300     IN      AAAA    ::1
ns1.asf.as.              300     IN      A       127.0.0.2
ns1.asf.as.              300     IN      AAAA    ::2

-> ns1 IN AAAA ::2

mail.asf.as.             300     IN      A       127.0.0.1
mail.asf.as.             300     IN      A       127.0.0.2

-> mail IN A 127.0.0.2