Systems manager uses a variety of uniquely identifying values from clients to attempt to determine a device's hardware identity for pairing against it's Systems Manager identity during enrollments or re-enrollments. These alerts are designed to alert you when Dashboard has determined that a device is, in fact, unique but important 'unique' identifiers aren't. On desktop machines, this is usually the BIOS UUID (Universally Unique Identifier). On mobile devices, this is usually the IMEI (International Mobile station Equipment Identity).
If Dashboard detects a collision of these values for enrolled or enrolling devices, an alert may be displayed with a link to filter the Clients list down to those devices which are suspect. These values are important for both SM and other software; on mobile devices, these values can affect the device's ability to connect to the cellular grid.
You can verify a non-unique UUID on desktop operating systems by running the following command(s). These commands should be run on each of the flagged clients to ensure that it's not a false positive.
wmic path win32_computersystemproduct get uuid
system_profiler SPHardwareDataType | grep "Hardware UUID:"
You can verify a non-unique IMEI value by comparing the About Device fields on devices to see if they have unique values. If the devices have the same IMEI value, it's recommended to contact the manufacturer or reseller of the device, as the shared IMEI will impact the device's ability to connect to the cellular grid.