Index: /trunk/src/org/openstreetmap/josm/command/AddPrimitivesCommand.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/command/AddPrimitivesCommand.java	(revision 17239)
+++ /trunk/src/org/openstreetmap/josm/command/AddPrimitivesCommand.java	(revision 17240)
@@ -100,6 +100,7 @@
             // a subsequent command (e.g. MoveCommand) cannot be redone.
             for (OsmPrimitive osm : createdPrimitives) {
-                if (preExistingData.stream().anyMatch(pd -> pd.getUniqueId() == osm.getUniqueId())) {
-                    Optional<PrimitiveData> o = data.stream().filter(pd -> pd.getUniqueId() == osm.getUniqueId()).findAny();
+                if (preExistingData.stream().anyMatch(pd -> pd.getPrimitiveId().equals(osm.getPrimitiveId()))) {
+                    Optional<PrimitiveData> o = data.stream()
+                            .filter(pd -> pd.getPrimitiveId().equals(osm.getPrimitiveId())).findAny();
                     if (o.isPresent()) {
                         osm.load(o.get());
@@ -126,6 +127,9 @@
             createdPrimitives = PurgeCommand.topoSort(createdPrimitives);
         }
-        for (OsmPrimitive osm : createdPrimitives) {
-            Optional<PrimitiveData> previous = preExistingData.stream().filter(pd -> pd.getUniqueId() == osm.getUniqueId()).findAny();
+        // reversed order, see #14620
+        for (int i = createdPrimitives.size() - 1; i >= 0; i--) {
+            OsmPrimitive osm = createdPrimitives.get(i);
+            Optional<PrimitiveData> previous = preExistingData.stream()
+                    .filter(pd -> pd.getPrimitiveId().equals(osm.getPrimitiveId())).findAny();
             if (previous.isPresent()) {
                 osm.load(previous.get());
Index: /trunk/test/unit/org/openstreetmap/josm/command/AddPrimitivesCommandTest.java
===================================================================
--- /trunk/test/unit/org/openstreetmap/josm/command/AddPrimitivesCommandTest.java	(revision 17239)
+++ /trunk/test/unit/org/openstreetmap/josm/command/AddPrimitivesCommandTest.java	(revision 17240)
@@ -187,9 +187,9 @@
         assertTrue(command.executeCommand());
 
-        assertEquals(3, ds.getNodes().size());
-        assertEquals(1, ds.getWays().size());
-
         for (int i = 0; i < 2; i++) {
             // Needs to work multiple times.
+            assertEquals(3, ds.getNodes().size());
+            assertEquals(1, ds.getWays().size());
+
             command.undoCommand();
 
@@ -199,7 +199,86 @@
             // redo
             assertTrue(command.executeCommand());
-
-            assertEquals(3, ds.getNodes().size());
-            assertEquals(1, ds.getWays().size());
+        }
+    }
+
+    /**
+     * Tests if the undo command does not remove
+     * data ignored by by the add command because they where already existing.
+     * Simulates regression in #14620
+     */
+    @Test
+    public void testUndoIgnoresExistingAsDeleted() {
+        DataSet ds = new DataSet();
+
+        List<PrimitiveData> testData = createTestData();
+
+        assertTrue(new AddPrimitivesCommand(testData, ds).executeCommand());
+        assertEquals(2, ds.getNodes().size());
+        assertEquals(1, ds.getWays().size());
+
+        ds.getNodes().forEach(n -> n.setDeleted(true));
+
+        AddPrimitivesCommand command = new AddPrimitivesCommand(testData, ds);
+
+        assertTrue(command.executeCommand());
+
+        for (int i = 0; i < 2; i++) {
+            // Needs to work multiple times.
+            assertEquals(2, ds.getNodes().size());
+            assertEquals(1, ds.getWays().size());
+
+            command.undoCommand();
+
+            assertEquals(2, ds.getNodes().size());
+            assertEquals(1, ds.getWays().size());
+
+            // redo
+            assertTrue(command.executeCommand());
+        }
+    }
+
+    /**
+     * Tests if the undo command does not remove
+     * data ignored by by the add command because they where already existing.
+     */
+    @Test
+    public void testUndoIgnoresExistingSameUniqueIdDifferentType() {
+        DataSet ds = new DataSet();
+
+        List<PrimitiveData> testData = new ArrayList<>(createTestData());
+
+        assertTrue(new AddPrimitivesCommand(testData, ds).executeCommand());
+        assertEquals(2, ds.getNodes().size());
+        assertEquals(1, ds.getWays().size());
+
+        NodeData n7Data = createTestNode(7);
+        NodeData n8Data = createTestNode(8);
+        Way w2 = new Way(5);
+        Node n7 = new Node(7);
+        n7.load(n7Data);
+        Node n8 = new Node(8);
+        n8.load(n8Data);
+        w2.setNodes(Arrays.asList(n7, n8));
+        testData.set(2, createTestNode(7));
+        testData.add(n8.save());
+        testData.add(w2.save());
+
+        AddPrimitivesCommand command = new AddPrimitivesCommand(testData, ds);
+
+        assertTrue(command.executeCommand());
+
+        for (int i = 0; i < 2; i++) {
+            assertEquals(4, ds.getNodes().size());
+            assertEquals(2, ds.getWays().size());
+
+            // Needs to work multiple times.
+            command.undoCommand();
+
+            assertEquals(2, ds.getNodes().size());
+            assertEquals(1, ds.getWays().size());
+
+            // redo
+            assertTrue(command.executeCommand());
+
         }
     }
@@ -266,5 +345,5 @@
         NodeData node1 = createTestNode(5);
         NodeData node2 = createTestNode(6);
-        WayData way = new WayData();
+        WayData way = new WayData(2);
         way.put("test", "test");
         way.setNodeIds(Arrays.asList(node1.getId(), node2.getId()));