If the clients have duplicate ID, one client will get registered and that info will be in DB, when other client tries to use the same ID, SEPM will reject as there is already an entry. Thats why you see green dot for a minute and then it disappears.
use this query to check the hwid in DB
SELECT * FROM SEM_CLIENT WHERE HARDWARE_KEY IN ( SELECT HARDWARE_KEY FROM SEM_CLIENT GROUP BY HARDWARE_KEY HAVING COUNT(HARDWARE_KEY) > 1) AND HARDWARE_KEY != '' ORDER BY HARDWARE_KEY;
This will show all machines where more than one hardware key is present in the database.