Thanks for you answer. I managed to make the value unique by inheriting the product module. This page explains how to write a costum module http://www.pixelite.co.nz/article/adding-additional-fields-using-custom-module-openerp-7.

Here the python file (if someone wants to do the same):

from openerp.osv import fields, osv

class unique_reference(osv.osv):

   _inherit = "product.product"

   _sql_constraints = [
        ('uniq_defaut_code', 'unique(default_code)', "A external reference already exists with this name . External reference must be unique!"),
   ]

unique_reference()

You need to add some custom code, inheriting the product module and adding a constraint to the field. That will make it unique, even with a small message telling the user their input should be unique.

By default I don't think there is any module that does this for you, although I would be happy to proven wrong.