execution: Allow a subgraph nodes to execute multiple times (#10499)

In the case of --cache-none lazy and subgraph execution can cause
anything to be run multiple times per workflow. If that rerun nodes is
in itself a subgraph generator, this will crash for two reasons.

pending_subgraph_results[] does not cleanup entries after their use.
So when a pending_subgraph_result is consumed, remove it from the list
so that if the corresponding node is fully re-executed this misses
lookup and it fall through to execute the node as it should.

Secondly, theres is an explicit enforcement against dups in the
addition of subgraphs nodes as ephemerals to the dymprompt. Remove this
enforcement as the use case is now valid.

execution.py

@@ -445,6 +445,7 @@ async def execute(server, dynprompt, caches, current_item, extra_data, executed,
                     resolved_outputs.append(tuple(resolved_output))
             output_data = merge_result_data(resolved_outputs, class_def)
             output_ui = []
+            del pending_subgraph_results[unique_id]
             has_subgraph = False
         else:
             get_progress_state().start_progress(unique_id)
@@ -527,10 +528,6 @@ async def execute(server, dynprompt, caches, current_item, extra_data, executed,
                 if new_graph is None:
                     cached_outputs.append((False, node_outputs))
                 else:
-                    # Check for conflicts
-                    for node_id in new_graph.keys():
-                        if dynprompt.has_node(node_id):
-                            raise DuplicateNodeError(f"Attempt to add duplicate node {node_id}. Ensure node ids are unique and deterministic or use graph_utils.GraphBuilder.")
                     for node_id, node_info in new_graph.items():
                         new_node_ids.append(node_id)
                         display_id = node_info.get("override_display_id", unique_id)