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
sign_partial_line = sign if partial_line.credit_move_id == line else (-1 * sign)
                amount += sign_partial_line * partial_line.amount
                
                if line.currency_id:
                    if partial_line.currency_id and partial_line.currency_id == line.currency_id:
                        amount_residual_currency += sign_partial_line * partial_line.amount_currency
                    else:
                        if line.balance and line.amount_currency:
                            rate = line.amount_currency / line.balance
                        else:
                            date = partial_line.credit_move_id.date if partial_line.debit_move_id == line else partial_line.debit_move_id.date
                            rate = line.currency_id.with_context(date=date).rate
                        amount_residual_currency += sign_partial_line * line.currency_id.round(partial_line.amount * rate)
            
            
            reconciled = False
            digits_rounding_precision = line.company_id.currency_id.rounding
            if float_is_zero(amount, precision_rounding=digits_rounding_precision):
                if line.currency_id and line.amount_currency:
                    if float_is_zero(amount_residual_currency, precision_rounding=line.currency_id.rounding):
                        reconciled = True
                else:
                    reconciled = True
            line.reconciled = reconciled
            line.amount_residual = line.company_id.currency_id.round(amount * sign)
            line.amount_residual_currency = line.currency_id and line.currency_id.round(amount_residual_currency * sign) or 0.0
    @api.depends('debit', 'credit')
    def _store_balance(self):