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)

if self.env.context.get('default_journal_type'):
            return self.env['account.journal'].search([('type', '=', self.env.context['default_journal_type'])], limit=1).id
    name = fields.Char(string='Number', required=True, copy=False, default='/')
    ref = fields.Char(string='Reference', copy=False)
    date = fields.Date(required=True, states={'posted': [('readonly', True)]}, index=True, default=fields.Date.context_today)
    journal_id = fields.Many2one('account.journal', string='Journal', required=True, states={'posted': [('readonly', True)]}, default=_get_default_journal)
    currency_id = fields.Many2one('res.currency', compute='_compute_currency', store=True, string="Currency")
    rate_diff_partial_rec_id = fields.Many2one('account.partial.reconcile', string='Exchange Rate Entry of', help="Technical field used to keep track of the origin of journal entries created in case of fluctuation of the currency exchange rate. This is needed when cancelling the source: it will post the inverse journal entry to cancel that part too.")
    state = fields.Selection([('draft', 'Unposted'), ('posted', 'Posted')], string='Status',
      required=True, readonly=True, copy=False, default='draft',
      help='All manually created new journal entries are usually in the status \'Unposted\', '
           'but you can set the option to skip that status on the related journal. '
           'In that case, they will behave as journal entries automatically created by the '
           'system on document validation (invoices, bank statements...) and will be created '
           'in \'Posted\' status.')
    line_ids = fields.One2many('account.move.line', 'move_id', string='Journal Items',
        states={'posted': [('readonly', True)]}, copy=True)
    partner_id = fields.Many2one('res.partner', related='line_ids.partner_id', string="Partner", store=True, readonly=True)
    amount = fields.Monetary(compute='_amount_compute', store=True)
    narration = fields.Text(string='Internal Note')
    company_id = fields.Many2one('res.company', related='journal_id.company_id', string='Company', store=True, readonly=True,
        default=lambda self: self.env.user.company_id)
    matched_percentage = fields.Float('Percentage Matched', compute='_compute_matched_percentage', digits=0, store=True, readonly=True, help="Technical field used in cash basis method")
    statement_line_id = fields.Many2one('account.bank.statement.line', string='Bank statement line reconciled with this entry', copy=False, readonly=True)
    
    dummy_account_id = fields.Many2one('account.account', related='line_ids.account_id', string='Account', store=False)
    @api.model
    def create(self, vals):