When you duplicate any document type, the COPY method is called.

To change how COPY works, write a new function like this:

def copy(self, default=None):
  default = dict(default or {})
  default.update({ '[field you want to change the default value of]': '[value you want]', })
  
  return super([class you are changing], self).copy(default)

default is a dictionary that tells Odoo that you want to use the contents as defaults for a new record (instead of the value already in the field from the record you are duplicating).

Also see http://stackoverflow.com/questions/20335480/what-really-happens-when-i-press-duplicate