Impacted versions:
10.0 + e

Steps to reproduce:
1) Create a salary rule that has 'appears_on_payslip' = False.
- An example would be a company contribution that the employee does not need to see.
  2) Add it to a salary structure
  3) run a payroll/payslip calculation

Current behavior:
Numerous thing.
1) Every time you compute sheet, you will duplicate these 'hidden' lines
2) details_by_salary_rule_category no longer has any of the hidden lines, though they certainly contribute to categories (e.g. tests will fail, and bad things)
3) when using hr_payroll_account, lines with contribution registers and accounts that SHOULD be on the journal entry are not !!

Expected behavior:
1) every time you compute the sheet, the lines should be deleted and recalculated
2) details_by_salary_rule_category should have all categories (to be able to make complex rules and tests)
3) journal entry should have a line for every line with an account

Pull Request eminent.

_inherit = 'hr.employee'
    _description = 'Employee'
    slip_ids = fields.One2many('hr.payslip', 'employee_id', string='Payslips', readonly=True)
    payslip_count = fields.Integer(compute='_compute_payslip_count', string='Payslips', groups="hr_payroll.group_hr_payroll_user")
    @api.multi
    def _compute_payslip_count(self):
        for employee in self:
            employee.payslip_count = len(employee.slip_ids)