Based on the information provided in the extracts, Ory Kratos supports two types of flows: flows where the user interacts with a browser-based application (either server-side or client-side), and flows where API interaction is required (like mobile apps or Smart TVs). These flows are used for various self-service operations like user login, user registration, profile management, account recovery, and email or phone verification. However, the extracts do not provide specific information on creating a unified flow for login and registration. The standard flows in Ory Kratos seem to treat login and registration as separate processes, each with their own endpoints and data models. For custom requirements, Ory Kratos does allow you to implement your own custom flows. But whether a unified login/registration flow can be implemented, and whether it would require a custom backend, is not specified in the provided extracts. For more detailed information, you may want to refer to the [Ory Kratos User Login](https://www.ory.sh/docs/kratos/self-service/flows/user-login) and [User Registration Documentation](https://www.ory.sh/docs/kratos/self-service/flows/user-registration) as mentioned in the extracts.

Based on the provided extracts, there isn't specific information on how to check if a user's email has already been registered with Ory Kratos. However, Ory Kratos does provide mechanisms for email verification and account activation, which implies that it keeps track of registered email addresses. For more detailed information on how to check if a user's email has already been registered, you may want to refer to the [Ory Kratos Documentation](https://www.ory.sh/docs/kratos/).