I was recently checking through Splunk when I came across these messages in the logs of one of my servers:

Oct 7 20:59:33 web named[13698]: client 111.222.333.444#50389: updating zone ‘domain.com/IN’: update failed: ‘RRset exists (value dependent)’ prerequisite not satisfied (NXRRSET)
Oct 7 20:59:34 web named[13698]: client 111.222.333.444#50392: update ‘domain.com/IN’ denied
Oct 7 21:59:35 web named[13698]: client 111.222.333.444#50422: updating zone ‘domain.com/IN’: update failed: ‘RRset exists (value dependent)’ prerequisite not satisfied (NXRRSET)
Oct 7 21:59:35 web named[13698]: client 111.222.333.444#50425: update ‘domain.com/IN’ denied
Oct 7 22:59:20 web named[13698]: client 111.222.333.444#50458: updating zone ‘domain.com/IN’: update failed: ‘RRset exists (value dependent)’ prerequisite not satisfied (NXRRSET)

A quick google sent me to this link (who I have gracefully stolen the logs from instead of bothering to redact my own), which told me:

The messages here are actually showing that named is doing its job well. Some user was attempting to dynamically update a DNS zone repeatedly, but named was rejecting the updates since they were not coming from a valid sources.

Problem solvered!