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
""" Automatically create a journal entry to book the exchange rate difference.
            That new journal entry is made in the company `currency_exchange_journal_id` and one of its journal
            items is matched with the other lines to balance the full reconciliation.
        """
        for rec in self:
            if not rec.company_id.currency_exchange_journal_id:
                raise UserError(_("You should configure the 'Exchange Rate Journal' in the accounting settings, to manage automatically the booking of accounting entries related to differences between exchange rates."))
            if not self.company_id.income_currency_exchange_account_id.id:
                raise UserError(_("You should configure the 'Gain Exchange Rate Account' in the accounting settings, to manage automatically the booking of accounting entries related to differences between exchange rates."))
            if not self.company_id.expense_currency_exchange_account_id.id:
                raise UserError(_("You should configure the 'Loss Exchange Rate Account' in the accounting settings, to manage automatically the booking of accounting entries related to differences between exchange rates."))
            move_vals = {'journal_id': rec.company_id.currency_exchange_journal_id.id, 'rate_diff_partial_rec_id': rec.id}
            
            
            
            if move_date > rec.company_id.fiscalyear_lock_date:
                move_vals['date'] = move_date
            move = rec.env['account.move'].create(move_vals)
            amount_diff = rec.company_id.currency_id.round(amount_diff)
            diff_in_currency = currency.round(diff_in_currency)
            line_to_reconcile = rec.env['account.move.line'].with_context(check_move_validity=False).create({
                'name': _('Currency exchange rate difference'),