How to restrict duplication of training enrollment to a specific course?

Oracle Learning Management- EBS 12.2.5

Suppose we have target population of 100 employees and we are planning to conduct their training in 5 session with max. attendance of 20 per session. In this scenario we have created all 5 five classes and give learner access this targeted population to give their nomination. But while provide nominations users are enrolling themselves in all 5 session. We want to restrict this duplication enrollment.