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
for line in self:
            line.balance = line.debit - line.credit
    @api.model
    def _get_currency(self):
        currency = False
        context = self._context or {}
        if context.get('default_journal_id', False):
            currency = self.env['account.journal'].browse(context['default_journal_id']).currency_id
        return currency
    @api.model
    def _get_journal(self):
        """ Return journal based on the journal type """
        context = dict(self._context or {})
        journal_id = context.get('journal_id', False)
        if journal_id:
            return journal_id
        journal_type = context.get('journal_type', False)
        if journal_type:
            recs = self.env['account.journal'].search([('type', '=', journal_type)])
            if not recs:
                action = self.env.ref('account.action_account_journal_form')
                msg = _("""Cannot find any account journal of "%s" type for this company, You should create one.\n Please go to Journal Configuration""") % journal_type.replace('_', ' ').title()
                raise RedirectWarning(msg, action.id, _('Go to the configuration panel'))
            journal_id = recs[0].id
        return journal_id
    @api.depends('debit', 'credit', 'move_id.matched_percentage', 'move_id.journal_id')
    def _compute_cash_basis(self):