I'm a bit stuck and wonder if someone can help me...

I have a form with a formula field which generates a customer reference:

Form name: Registration form

Single line: Last name

Date: Date of birth

Formula: Customer reference ((((Date_of_birth.getMonth())) + Last_name) + (Date_of_birth.getDay())) + (Date_of_birth.getYear())

I'd like to set it so that if someone tries to fill in the form again (and therefore generates the same customer reference) an error message will show e.g. 'you are already registered'. 

I also have another form where a registered customer can book a service. I'd like to set it so that they have to type in their customer reference number into a single line field to have it validated before they can book the service. 

If it matches a customer reference already stored in the Registration form then it's validated and they can go on to fill in the rest of the form. If it does not match an existing customer reference number then an error message will show e.g. 'you are not yet a registered customer. Please complete a customer registration form'. 

We will have very few customers so the likelihood of someone with the same last name and same date of birth is low so I'd prefer to perform these actions with the formula field if possible. If not, I could use the unique Zoho ID generated for each record. 

Anyone have any bright ideas? 

Thank you!

Lindsay