This actually goes a notch deeper. In MobileConnect your Contacts have a priority
on them, which means you can never have two (or more) Contacts with priority 1
status.
By default a new entry with an existing Contacts mobile number will be set as priority 1
Contact and everyone else sharing this number (as their primary number) will be set as priority 2
.
This means any personalization strings or lookups based on ContactKey
or Mobile_Number
you do in your SMS are based on the Contact having priority 1
status.
So while it's correct MobileConnect 'deduplicates' for an SMS send, it's actually based on the priority
level - not the mobile number itself.