Index: /trunk/src/org/openstreetmap/josm/data/cache/JCSCachedTileLoaderJob.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/cache/JCSCachedTileLoaderJob.java	(revision 15572)
+++ /trunk/src/org/openstreetmap/josm/data/cache/JCSCachedTileLoaderJob.java	(revision 15573)
@@ -7,5 +7,4 @@
 import java.net.URL;
 import java.security.SecureRandom;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -156,5 +155,5 @@
             first = !inProgress.containsKey(deduplicationKey);
         }
-        inProgress.computeIfAbsent(deduplicationKey, k -> new HashSet<>()).add(listener);
+        inProgress.computeIfAbsent(deduplicationKey, k -> ConcurrentHashMap.newKeySet()).add(listener);
 
         if (first || force) {