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)

currency = list(partial_rec_set)[0].currency_id
        maxdate = None
        aml_to_balance = None
        for partial_rec in partial_rec_set:
            if partial_rec.currency_id != currency:
                
                currency = None
            for aml in [partial_rec.debit_move_id, partial_rec.credit_move_id]:
                if aml not in aml_set:
                    if aml.amount_residual or aml.amount_residual_currency:
                        aml_to_balance = aml
                    maxdate = max(aml.date, maxdate)
                    total_debit += aml.debit
                    total_credit += aml.credit
                    aml_set |= aml
                    if aml.currency_id and aml.currency_id == currency:
                        total_amount_currency += aml.amount_currency
                    elif partial_rec.currency_id and partial_rec.currency_id == currency:
                        
                        
                        
                        total_amount_currency += aml.company_id.currency_id.with_context(date=aml.date).compute(aml.balance, partial_rec.currency_id)
                for x in aml.matched_debit_ids | aml.matched_credit_ids:
                    partial_rec_set[x] = None
        
        if currency and aml_to_balance and (float_is_zero(total_amount_currency, precision_rounding=currency.rounding) or float_compare(total_debit, total_credit, precision_rounding=currency.rounding) == 0):
            
            aml_id, partial_rec_id = partial_rec.create_exchange_rate_entry(aml_to_balance, total_debit - total_credit, total_amount_currency, currency, maxdate)