I recommend you use purging task from both the system tree and the rsd tables. just to be sure please check if you run the following tasks regularly:
- inactive agent cleanup task
- duplicate agent guid detection with blacklisting (aka high sequence clients)
- duplicate node deletion (plain)
- rsd automated response with push agent and/or delete detected system (alternatively: exclude from detection in the case of non-Windows hosts)
If you see duplicates in reports, please check the agent GUId of them to see if only the hostname matches (AgentGUID only matches if they are the same host), and also the last ASCI time to see if one of them is not an inactive agent.
Not sure entirely, but duplicates could be due to VPN users, this could be the last you check. A possible remedy is to add VPN vendor code into the ePO database so VPN hosts are treated irrespective of their - common - MACs.