validate_trigger_paths used seen_paths.get(trigger_path) and treated a None
result as "path not seen yet". But None is also what node.get("id") returns
for a node without an id, so when the first trigger node sharing a path had no
id, it was stored as None and every later node with the same path was silently
accepted as unique — duplicate trigger paths slipped through validation.
Use a membership test (trigger_path not in seen_paths) so "first occurrence"
and "node_id happens to be None" are no longer conflated. Behavior is
unchanged for nodes that have ids.
Adds a regression test that fails before and passes after.
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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