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
writeoff_vals = {
                'account_id': writeoff_acc_id.id,
                'journal_id': writeoff_journal_id.id
            }
            if not all_aml_share_same_currency:
                writeoff_vals['amount_currency'] = False
            writeoff_to_reconcile = remaining_moves._create_writeoff(writeoff_vals)
            
            remaining_moves = (remaining_moves + writeoff_to_reconcile).auto_reconcile_lines()
            return writeoff_to_reconcile
        return True
    def _create_writeoff(self, vals):
        """ Create a writeoff move for the account.move.lines in self. If debit/credit is not specified in vals,
            the writeoff amount will be computed as the sum of amount_residual of the given recordset.
            :param vals: dict containing values suitable fot account_move_line.create(). The data in vals will
                be processed to create bot writeoff acount.move.line and their enclosing account.move.
        """
        
        if 'account_id' not in vals or 'journal_id' not in vals:
            raise UserError(_("It is mandatory to specify an account and a journal to create a write-off."))
        if ('debit' in vals) ^ ('credit' in vals):
            raise UserError(_("Either pass both debit and credit or none."))
        if 'date' not in vals:
            vals['date'] = self._context.get('date_p') or time.strftime('%Y-%m-%d')
        if 'name' not in vals:
            vals['name'] = self._context.get('comment') or _('Write-Off')
        
        if 'credit' not in vals and 'debit' not in vals:
            amount = sum([r.amount_residual for r in self])
            vals['credit'] = amount > 0 and amount or 0.0