When Failed actually means, "Already exists!"

When Failed actually means, "Already exists!"

We have set up a flow so that when Zoho Bookings has a new appointment made, it also creates a new customer in our Shopify shop customer list, so when they attend the details are already in the POS..
This works fine for new customers...
however...
If the flow encounters the same email or phone number, rather than just ignoring it, ( as expected as the customer already exists in Shopify and does not need adding) it says "Failed". 
I got an automatic email saying it had failed 3 times in a row, and if after 20 failures it will switch off! 
What can I do to tell the flow that if a customer already exists, it OK? and do nothing?
Operationally it OK but at some point we will have 20 fails and it will switch off, until I notice!
Thanks
Barry