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
result = []
        for line in self:
            if line.ref:
                result.append((line.id, (line.move_id.name or '') + '(' + line.ref + ')'))
            else:
                result.append((line.id, line.move_id.name))
        return result
    @api.model
    def compute_amount_fields(self, amount, src_currency, company_currency, invoice_currency=False):
        """ Helper function to compute value for fields debit/credit/amount_currency based on an amount and the currencies given in parameter"""
        amount_currency = False
        currency_id = False
        if src_currency and src_currency != company_currency:
            amount_currency = amount
            amount = src_currency.with_context(self._context).compute(amount, company_currency)
            currency_id = src_currency.id
        debit = amount > 0 and amount or 0.0
        credit = amount < 0 and -amount or 0.0
        if invoice_currency and invoice_currency != company_currency and not amount_currency:
            amount_currency = src_currency.with_context(self._context).compute(amount, invoice_currency)
            currency_id = invoice_currency.id
        return debit, credit, amount_currency, currency_id
    @api.multi
    def create_analytic_lines(self):