Index: trunk/src/org/openstreetmap/josm/gui/dialogs/properties/AbstractCopyAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/properties/AbstractCopyAction.java	(revision 15812)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/properties/AbstractCopyAction.java	(revision 15813)
@@ -49,5 +49,6 @@
                 .flatMap(key -> sel.stream().map(p -> getString(p, key)))
                 .filter(Objects::nonNull)
-                .flatMap(Collection::stream);
+                .flatMap(Collection::stream)
+                .distinct();
     }