Fail validation of frontend form if field value is not unique

I was surprised to see that there is no plugin in the store for the purpose of requiring unique values on fields. I need to create a form that will not let front end users create an account if they enter the same [identifying number] as another user.

Is there no way of configuring custom validation logic for creating an entry, and/or for making a field require a unique value, or am I going to finally be forced into learning how to write a module here?