Index: /trunk/src/org/openstreetmap/josm/io/AbstractParser.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/AbstractParser.java	(revision 18956)
+++ /trunk/src/org/openstreetmap/josm/io/AbstractParser.java	(revision 18957)
@@ -186,7 +186,5 @@
         RelationMemberData member = new RelationMemberData(role, type, ref);
         // see #20405: cache equal instances of members
-        RelationMemberData cachedMember = memberCache .putIfAbsent(member, member);
-        if (cachedMember == null)
-            cachedMember = member;
+        RelationMemberData cachedMember = memberCache.computeIfAbsent(member, m -> m);
         ((HistoryRelation) currentPrimitive).addMember(cachedMember);
     }