Mail delivery
MX records are distinct
mail.mx-duplicates
Duplicate hosts add no redundancy, and duplicate preferences make delivery order arbitrary.
A longer write-up of this check has not been published yet. The report itself shows the raw evidence behind every result, which is usually the fastest way to see what was measured.
Run this check on a domain
MX records are distinct is one of 54 checks in every report, alongside delegation, mail authentication, TLS and registration.