Index: src/org/openstreetmap/josm/actions/JoinAreasAction.java
===================================================================
--- src/org/openstreetmap/josm/actions/JoinAreasAction.java	(revision 15887)
+++ src/org/openstreetmap/josm/actions/JoinAreasAction.java	(working copy)
@@ -62,6 +62,7 @@
     private DataSet ds;
     private final transient List<Relation> addedRelations = new LinkedList<>();
     private final boolean addUndoRedo;
+    private boolean tagConflictsResolved;
 
     /**
      * This helper class describes join areas action result.
@@ -484,6 +485,7 @@
 
     private void clearFields() {
         ds = null;
+        tagConflictsResolved = false;
         cmdsCount = 0;
         cmds.clear();
         addedRelations.clear();
@@ -570,6 +572,12 @@
                         allWays.addAll(pol.innerWays);
                     }
                     ds.setSelected(allWays);
+                    if (result.polygons.size() > 1 && tagConflictsResolved) {
+                        new Notification(tr("Selected ways were not joined to a single area but have the same tags now."
+                                + "<br>Please verify no errors have been introduced."))
+                                        .setIcon(JOptionPane.INFORMATION_MESSAGE).setDuration(Notification.TIME_LONG)
+                                        .show();
+                    }
                 }
             } else {
                 new Notification(
@@ -816,6 +824,7 @@
         TagCollection wayTags = TagCollection.unionOfAllPrimitives(ways);
         try {
             cmds.addAll(CombinePrimitiveResolverDialog.launchIfNecessary(wayTags, ways, ways));
+            tagConflictsResolved = (!cmds.isEmpty());
             commitCommands(marktr("Fix tag conflicts"));
             return true;
         } catch (UserCancelException ex) {