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Thank you for reaching the community forum.
For your first query, it is yes, after 90 days of being offline, the license will automatically remove from the device, and you’ll have one available license to use.
For the next query, it isn’t advisable to use a duplicate hostname for your devices as this will cause conflict on your Sophos central, and your device may not update properly.
You don't need to reinstall the endpoint on the computer you wish to retain. But you need to delete the duplicate hostname on Sophos Central. You can Identify them as the device's status will show if they're online on not.
Thank you for getting back to us.
If the devices go online after day 91, will Sophos be uninstalled? - No, the endpoint will remain on the device. However, it won't be updating to Sophos Central
The number of licenses is the same as the number of devices with the green checks icon on the devices tab in Sophos Central]
Is this correct? - No, Endpoint client licensing depends on users. You may refer to this KB article to further understand the license distribution for your clients.