Error Duplicate record found in rrset: example.tld when modifying a zone

Symptoms

The following error is thrown when modifying a DNS zone:

Error TTL mismatch in rrset: example.tld

Cause

A given record appears twice in the zone. For example:

example.tld A 1.2.3.4
example.tld A 1.2.3.4

Resolution

Remove duplicate records.