sonata-project/SonataAdminBundle — pull request #758 by sjopet (2012-05-16 14:49:39+00:00)
Title: added a condition for 'add new' button 

so the button will only display when you edit an entity and no longer is displayed when you are creating a new instance of the entity.

The 'create new' button on the create form causes problems when using a 'flow' form.
When you override the create function in an admin's controller and use the admin's getPersistentParameters and getNewInstance functions. You can have additional forms a user needs to fill before displaying the original create form.
Because the admin's persistent parameters are filled based on request parameters the 'add new' button on a create form won't work properly, since the request parameters are still there you won't get redirected to the additional form.

In addition I don't think it makes much sense having the 'add new' button on a form while you are creating a new item.