If the device is discovered with Agent based discovery, and if the agent is reinstalled in end machine due to system got formatted/restored for any reason and it will discover as a new system and it will create duplicate system entries.
To clear duplicate systems, conflict merge operation to be conducted from ‘Settings > Node Operations > Conflicted Systems’ Page. Refer below link for more information about conflict merging.
Please refer online help link for more details - Managed and Unmanaged Node
There is similar article published with respect to agent and agentless discovery, please refer link Customer has discovered one system with Agent-less. Later they have discovered same machine with Agent. Now they don't want to see two machines in SapphireIMS in any views or reports. What to be done for this scenario? if it helps.