Overview
Shopify permits only unique customer records based on an email or phone number. Some POS platforms enable multiple customers to share the same email or phone number.
There is currently no predictable way to manage 1-to-many customer matching, so Mortar links to the first matching customer record found in your POS when customers share the same email or phone number.
Step-by-step
Fortunately, fixing duplicate customer records in your POS is easy, you can:
Update each duplicate customer to make their contact information unique, or
Archive or Merge duplicate customers into one unique record for that customer
Learn more: Merging and Archiving customers