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)

addons/account/models/account_move.py
if writeoff_currency != company_currency:
                    mv_line_dict['debit'] = writeoff_currency.compute(mv_line_dict['debit'], company_currency)
                    mv_line_dict['credit'] = writeoff_currency.compute(mv_line_dict['credit'], company_currency)
                writeoff_lines += self._create_writeoff(mv_line_dict)
            (self + writeoff_lines).reconcile()
        else:
            self.reconcile()
    
    
    
    def _get_pair_to_reconcile(self):
        
        field = self[0].account_id.currency_id and 'amount_residual_currency' or 'amount_residual'
        rounding = self[0].company_id.currency_id.rounding
        if self[0].currency_id and all([x.amount_currency and x.currency_id == self[0].currency_id for x in self]):
            
            field = 'amount_residual_currency'
            rounding = self[0].currency_id.rounding
        
        sorted_moves = sorted(self, key=lambda a: a.date)
        debit = credit = False
        for aml in sorted_moves:
            if credit and debit:
                break
            if float_compare(aml[field], 0, precision_rounding=rounding) == 1 and not debit:
                debit = aml
            elif float_compare(aml[field], 0, precision_rounding=rounding) == -1 and not credit:
                credit = aml
        return debit, credit
    def auto_reconcile_lines(self):