Closing as session twice should not be an error if we can avoid it; the guidance in Closeable is that close() should be idempotent.
Nevertheless it's useful to have more debug info to understand when this happens (it really shouldn't but was observed, see stacktrace).
2018-10-25 12:15:42,420 [scrollerPool-00,in:1,inCheckpoint:1,out:1,lastRead:1540469741419,lastTimer:0,wm:201912449935343617,loop:15,checkpoint] ERROR [ComputationRunner] scroller: Exception in processLoop: Closing unknown CoreSession: test/Administrator#194 java.lang.RuntimeException: Closing unknown CoreSession: test/Administrator#194 at org.nuxeo.ecm.core.api.CoreSessionServiceImpl.releaseCoreSession(CoreSessionServiceImpl.java:53) ~[nuxeo-core-10.3-SNAPSHOT.jar:?] at org.nuxeo.ecm.core.api.CoreInstance.closeCoreSession(CoreInstance.java:133) ~[nuxeo-core-api-10.3-SNAPSHOT.jar:?] at org.nuxeo.ecm.core.api.local.LocalSession.close(LocalSession.java:154) ~[nuxeo-core-10.3-SNAPSHOT.jar:?] at org.nuxeo.ecm.core.bulk.computation.BulkScrollerComputation.processRecord(BulkScrollerComputation.java:148) ~[nuxeo-core-bulk-10.3-SNAPSHOT.jar:?] at org.nuxeo.ecm.core.bulk.computation.BulkScrollerComputation.lambda$processRecord$0(BulkScrollerComputation.java:104) ~[nuxeo-core-bulk-10.3-SNAPSHOT.jar:?] at org.nuxeo.runtime.transaction.TransactionHelper.lambda$runInTransaction$3(TransactionHelper.java:569) ~[nuxeo-runtime-jtajca-10.3-SNAPSHOT.jar:?]