HI,
Sorry for the typo. It should be Windows 8.1 not Windows 8. However, I have already confirmed that Windows 8.1 is not supported of version 12.1.2015.2015 upon checking on the release note. Furthermore, we've checked that version 12.1.6 MP3 is capable with Windows 8.1. See attached link for your reference.
Version 12.1.2015.2015 (Windows 8.1 not supported)
Version 12.1.6 MP3 (Window 8.1 supported)
And regarding on the duplicate license issue, I have tried again to double-check and re-count the clients but the problem is still exist. Based in total installed clients from our Symantec health alert notification (figure 2) is not same result with deployment summary (figure 1).
Since upgrading of SEPM is the only option to solve this issues. We would like to ask if we plan to upgrade or migrate our SEPM ver12.1.2015.2015 in latest version. Is it free or shall we need to purchase it?. and what upgrade process shall we need to be consider?