Repeated validations, while not DRY, aren't a case of coupling. They seem more a case of de-coupling, as they're needed for the application and the service laye

Repeated validations, while not DRY, aren't a case of coupling. They seem more a case of de-coupling, as they're needed for the application and the service layer to work independently from each other. While in non-spas your server receives from the client an extremely specific blob of page state plus user input and has to rebuild a specific new page with state updates and/or error messages.