Install the Report Manager extension

Once you have the report manager installed you can find it by clicking the admin tab > extras blocks at the bottom > report manager

Build a new report with the following settings:
Aggregation Type: Grouped
Report Type: Session
Group Fields: GuestMachineSerialNumber
Select Fields: Count
Filter: SessionType = 'Access' AND NOT IsEnded
Aggregate Filter: Count >1
Item Limit: Currently set at 10,000 but we only have ~ 900 devices so doesn't matter

Save it and then at the top is run options you can either download as a csv or get a download link

Best solution I have for this so far is to have a report that is based on duplicate serial numbers - whenever I see device count getting above what I would typically see as normal I run the report and pull the old devices. Having an option to replace if serial numbers match would be awesome