hi,
this can be because
1. Does the user login to an application , if so is there a restriction set in the application ?
2. check the users .profile , and global profile in /etc
to see if it checks the number of times a user has logged in and if more than once , exit
3. as solaix mentioned ,licenses check how many you have
# lslicense
4. Is this problem just with a particular user ?
Can you login multiple times as root ?
HTH