Impacted versions: 9.0, 9.0 enterprise

Steps to reproduce:
On current runbot duplicate any posted journal entry which is balanced

Current behavior:
Error message "Cannot create unbalanced entry" is shown

Expected behavior:
New Journal entry is created in state 'draft'

Video/Screenshot link (optional):
![selection_268](https://cloud.githubusercontent.com/assets/14974044/15007437/41f45a56-11d9-11e6-84f3-b297befaa521.png)

for move in self:
            if not move.journal_id.update_posted:
                raise UserError(_('You cannot modify a posted entry of this journal.\nFirst you should set the journal to allow cancelling entries.'))
        if self.ids:
            self._cr.execute('UPDATE account_move '\
                       'SET state=%s '\
                       'WHERE id IN %s', ('draft', tuple(self.ids),))
            self.invalidate_cache()
        return True
    @api.multi
    def unlink(self):
        for move in self:
            
            move.line_ids._update_check()
            move.line_ids.unlink()
        return super(AccountMove, self).unlink()
    @api.multi
    def _post_validate(self):
        for move in self:
            if move.line_ids:
                if not all([x.company_id.id == move.company_id.id for x in move.line_ids]):
                    raise UserError(_("Cannot create moves for different companies."))
        self.assert_balanced()
        return self._check_lock_date()
    @api.multi
    def _check_lock_date(self):