[add]: loan max amount

app/models/customer.py

@@ -50,6 +50,8 @@ class Customer(db.Model):
             raise ValueError("Customer already exists")
         elif Account.query.filter_by(id=account_id).first():
             raise ValueError("Account already exists")
+        elif cls.query.filter_by(msisdn=msisdn).first():
+            raise ValueError("msisdn already exists")
         
         # Create the customer
         customer = cls(