Matching
Each client has one stable identity, taken from their email address where there is one, and their phone number otherwise. Booking again with the same email or phone adds to the existing record.
When someone changes email
Update the email on the client record. Their past appointments, spend and messages move with them — the history is not split.
Why duplicates still happen
Usually a typo, or the same person booking once with an email and once with only a phone number. Correct the details on one record and future bookings match correctly.
Phone-only clients
A client with no email address is supported and keeps a full history. They cannot receive email confirmations or reminders until you add one.