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.