Prior precheck only compared the JSONL payload's ids against the live
doc — it didn't detect duplicates within the payload itself. A model
emitting the same id twice (two roots sharing an id, a nested child
reusing a root id, etc) would still slip past: addNode appends both
copies, getNodeById returns the first match for both verifies, and a
later rollback removeNode would delete only one of the two duplicates,
leaving an orphan with the same id in the doc.
Detect internal duplicates during the same id-collection pass:
collectIds tracks `treeIds` as a Set and pushes any id seen twice into
`internalDuplicates`. If non-empty, return `failed` immediately with
the duplicate list — same shape as the existing live-doc collision
branch, no doc mutation, no side effects, retry path takes over.
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