Having a dual MVC is absolutely redundant if your web frontend is simply the presentational layer ie. a view and all your data as well as application logic resides on the server.
However many of the modern complex web apps try to maximize the user experience by creating highly interactive frontends that dynamically communicate with the server using Ajax or similar alternatives eg. Flash AMF.
In such cases in the fontend of your application, separating out the script portions that handle actual communication with server, providing proper provisions for managing the local data that has been fetched/cached in the client's system, user interaction event handling and history management. Once you begin to think about it, it quickly becomes apparent that having a separate MVC layer in the javascript code is a good idea because it fits well with the scenario and makes the code manageable.
For instance, in an application like Facebook a user interaction event like pressing L when a picture is enlarged, or clicking on the Like button actually map to the same action and therefore this action should be decoupled from the part that constructs the view and attaches event handlers. Actual sending the updated metadata information to the server can be again separated out because this portion can be re-utilized for any action that updates the metadata which needs to be communicated back to the server. Similarly validation of metadata is reusable across different user actions which update the metadata.
Through this example I hope to have communicated the concept of how MVC design fits in with web frontends.
