Index: src/java/org/apache/lucene/search/FieldDocSortedHitQueue.java
===================================================================
--- src/java/org/apache/lucene/search/FieldDocSortedHitQueue.java	(revision 326129)
+++ src/java/org/apache/lucene/search/FieldDocSortedHitQueue.java	(working copy)
@@ -157,6 +157,11 @@
 				c = -c;
 			}
 		}
+
+    // avoid random sort order that could lead to duplicates (bug #31241):
+    if (c == 0)
+      return docA.doc > docB.doc;
+
 		return c > 0;
 	}
 }