only allow topological node consolidation to be run once on a graph

osmnx/_osm_xml.py

@@ -228,9 +228,9 @@ def _save_graph_xml(
         )
         warn(msg, category=UserWarning, stacklevel=2)
 
-    # raise error if graph has been simplified
-    if G.graph.get("simplified", False):
-        msg = "Graph must be unsimplified to save as OSM XML."
+    # raise error if graph has been simplified or nodes consolidated
+    if G.graph.get("simplified", False) or G.graph.get("consolidated", False):
+        msg = "Graph must be unsimplified and unconsolidated to save as OSM XML."
         raise GraphSimplificationError(msg)
 
     # set default filepath if None was provided

osmnx/io.py

@@ -188,7 +188,10 @@ def load_graphml(
         raise ValueError(msg)
 
     # specify default graph/node/edge attribute values' data types
-    default_graph_dtypes = {"simplified": _convert_bool_string}
+    default_graph_dtypes = {
+        "consolidated": _convert_bool_string,
+        "simplified": _convert_bool_string,
+    }
     default_node_dtypes = {
         "elevation": float,
         "elevation_res": float,

osmnx/simplification.py

@@ -630,6 +630,10 @@ def _consolidate_intersections_rebuild_graph(  # noqa: C901,PLR0912,PLR0915
         A rebuilt graph with consolidated intersections and (optionally)
         reconnected edge geometries.
     """
+    if G.graph.get("consolidated"):  # pragma: no cover
+        msg = "This graph has already been consolidated, cannot consolidate it again."
+        raise GraphSimplificationError(msg)
+
     # default node attributes to aggregate upon consolidation
     if node_attr_aggs is None:
         node_attr_aggs = {"elevation": "mean"}
@@ -715,6 +719,9 @@ def _consolidate_intersections_rebuild_graph(  # noqa: C901,PLR0912,PLR0915
                     node_attrs[col] = unique_vals
             Gc.add_node(cluster_label, **node_attrs)
 
+    # mark the graph as having been consolidated
+    G.graph["consolidated"] = True
+
     if len(G.edges) == 0 or not reconnect_edges:
         # if reconnect_edges is False or there are no edges in original graph
         # (after dead-end removed), then skip edges and return new graph as-is