'IllegalStateException: UT000196: Session with id already exists' error seen after upgrading from EAP 6 to EAP 7

Issue

• We recently upgraded from EAP 6 to EAP 7. Then in some cases, we see this exception:

ERROR [io.undertow.request] (default task-6) UT005023: Exception handling request to /page: java.lang.IllegalStateException: UT000196: Session with id 1M6JVjMTJsnlzZE9w4TKIJfGPTQ6pkXRVU66gnUW already exists
    at io.undertow.server.session.InMemorySessionManager.createSession(InMemorySessionManager.java:180)
    at io.undertow.servlet.spec.ServletContextImpl.getSession(ServletContextImpl.java:932)
    at io.undertow.servlet.spec.ServletContextImpl.getSession(ServletContextImpl.java:948)
    at io.undertow.servlet.handlers.security.ServletFormAuthenticationMechanism.storeInitialLocation(ServletFormAuthenticationMechanism.java:198)