Process instances are automatically reloaded when necessary. So that means the process instance will continue where it left off if it triggered again. For example, if you complete some task requested by that process instance, send it a signal, etc.
You can find numerous examples in the junit tests, e.g.
https://github.com/krisv/jbpm/blob/master/jbpm-persistence-jpa/src/test/java/org/jbpm/persistence/session/PersistentStatefulSessionTest.java
Kris