Create a query (name SysDup), Mananged Systems, under filter/System Name: Is duplicated and Last Communication: Is not within the last 3 days (or whatever). Save/Run and confirm results.
If results are good, create server task, under actions: run query, choose the Sysdup with sub-action delete systems and a schedule of your choosing (hourly, daily, weekly ...)
If this has been helpful/answered mark it. This will help others when searching the forum.