It happens easily: you book as a guest with one address, register later with another, and end up with your history split in two. Neither account is wrong, they simply do not know about each other.
There is no button for merging them, on purpose — a merge moves personal history between accounts and is not something that should be possible on the strength of typing an address. Contact us from one of the two addresses, tell us the other, and say which one you want to keep. We will confirm from the second address before moving anything.
What moves is your appointment history and your saved professionals. Your Beauty Passport is the part to check afterwards: where both accounts have entries for the same thing, keeping the fuller version and deleting the duplicate is a manual pass, and it is quicker for you to do than for anybody to guess at.
In the meantime, use one address consistently. A third account makes the eventual clean-up worse.