Any police department with an internal affairs division makes sure it is comprised of police officers who handle those cases exclusively. There is no conflict of interest if the investigator has no involvement - directly or indirectly - with any other area of police work. The same goes for the private sector. If any investigation is being done internally by human resources, no one on that team should be involved in any other section of that company. If the HR director takes on added responsibilities for the company’s president and there’s a complaint filed against the president, then the HR director has a glaring conflict and can’t be involved in the investigation. Avoid conflicts of interest at all costs. Any whiff of one means you need someone else to take over the investigation, even if it's an outside party.