This is by design.  The "Duplicate" in Action menu is calling the copy() method and direct to edit enable view mode.

To accomplish what you like, you could create a custom button with corresponding function.

This function should return a window action to form view with context and default_field.

Should look like this

def action_duplicate(self):
    self.ensure_one()
    context = {
        'default_field_1': self.field_1,
        'default_field_2': self.field_2,
        'default_field_3': self.field_3,
        ......
        ......
        ...... and so on    
        }
    return {
        'type': 'ir.actions.act_window',
        'view_type': 'form', 
        'view_mode': 'form', 
        'res_model': 'model.name', 
        'views': [(self.env.ref('module_name.form_view_xml_id').id, 'form')],
        'view_id': self.env.ref('module_name.form_view_xml_id').id,
        'target': 'current', 
        'context': context,        
    }

If you hit save, record is duplicated, discard will have nothing happen since you only open a form with default value.