diff --git a/mapmaker/flatmap/__init__.py b/mapmaker/flatmap/__init__.py
index 7424a482..fadafbd4 100644
--- a/mapmaker/flatmap/__init__.py
+++ b/mapmaker/flatmap/__init__.py
@@ -288,17 +288,15 @@ def get_feature_by_geojson_id(self, geojson_id: int) -> Optional[Feature]:
     #=========================================================================
         return self.__features_by_geojson_id.get(geojson_id)
 
-    def new_feature(self, layer_id: str, geometry, properties, is_group=False) -> Feature:
-    #=====================================================================================
+    def new_feature(self, layer_id: str, geometry, properties, is_group=False, update_feature_with_id=False) -> Feature:
+    #===================================================================================================================
         self.__last_geojson_id += 1
         properties['layer'] = layer_id
         self.properties_store.update_properties(properties)   # Update from JSON properties file
         feature = Feature(self.__last_geojson_id, geometry, properties, is_group=is_group)
         self.__features_by_geojson_id[feature.geojson_id] = feature
         if feature.id:
-            if feature.id in self.__features_with_id:
-                pass
-            else:
+            if update_feature_with_id or feature.id not in self.__features_with_id:
                 self.__features_with_id[feature.id] = feature
         if self.map_kind == MAP_KIND.FUNCTIONAL:
             if (name := properties.get('name', '')) != '':
diff --git a/mapmaker/flatmap/layers.py b/mapmaker/flatmap/layers.py
index 93960bf3..a4f5fff0 100644
--- a/mapmaker/flatmap/layers.py
+++ b/mapmaker/flatmap/layers.py
@@ -402,7 +402,7 @@ def add_group_features(self, group_name: str, features: list[Feature],
                     region_properties = base_properties.copy()
                     for region in filter(lambda p: prepared_polygon.contains(p.geometry), regions):
                         region_properties.update(region.properties)
-                        layer_features.append(self.flatmap.new_feature(self.id, polygon, region_properties))
+                        layer_features.append(self.flatmap.new_feature(self.id, polygon, region_properties, update_feature_with_id=True))
                         break
         else:
             for feature in features:
@@ -442,7 +442,7 @@ def add_group_features(self, group_name: str, features: list[Feature],
                 feature_group = self.flatmap.new_feature(
                         self.id,
                         shapely.MultiPolygon(grouped_polygons).buffer(0),
-                        grouped_properties, is_group=True)
+                        grouped_properties, is_group=True, update_feature_with_id=True)
                 layer_features.append(feature_group)
                 # So that any grouped lines don't have a duplicate id
                 grouped_properties.pop('id', None)
@@ -459,7 +459,7 @@ def add_group_features(self, group_name: str, features: list[Feature],
                 feature_group = self.flatmap.new_feature(
                       self.id,
                       shapely.MultiLineString(grouped_lines),
-                      grouped_properties, is_group=True)
+                      grouped_properties, is_group=True, update_feature_with_id=True)
                 layer_features.append(feature_group)
 
         # Feature specific properties have precedence over group's