[add]: loan ref to repayment

app/models/customer.py

@@ -48,6 +48,8 @@ class Customer(db.Model):
     def create_customer(cls, id, msisdn, country_code, account_id, account_type='savings'):
         if cls.query.filter_by(id=id).first():
             raise ValueError("Customer already exists")
+        elif Account.query.filter_by(id=account_id).first():
+            raise ValueError("Account already exists")
         
         # Create the customer
         customer = cls(