One Concurrent User Per Schema

1- Set in init.ora or spfile.ora the parameter
SQL@ homedev> alter system set resource_limit=true;
System altered.

2-  Connect to Database as SYSTEM or SYS and create the following Profile:
create profile single_user limit sessions_per_user 1;
3- Create the user/schema where only one concurrent connection should be allowed and grant other needed privileges.
create user test1 identified by test1 profile single_user;
grant connect, resource to test1;
4- Test it by trying to connecting with two users "test1":
SQL@ > conn test1/test1@homedev
Connected.
SQL@ > conn test1/test1@homedev
ERROR:
ORA-02391: exceeded simultaneous SESSIONS_PER_USER limit