he/him 
When writing Babel plugins, what can I assume about the same node being processed more than once? When or how can this happen? What are common protections if I don't want my annotations to appear twice? Should I always check if I've "seen" this node already?
You should always check if you’ve seen it. A WeakSet is the best option. There’s literally no memorable logic between when nodes are retraversed.
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Does path object have a shared lifetime with its node? Is path a bad key?
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Paths are associated with a parent and a key, the node object is fairly irrelevant. You should usually get the same Path instance for a given position, similar to React I think!
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@sebmck what scenario will get you to process the same node again? -
Another plugin replacing a parent will trigger a retraversal of the subtree, running your visitor again. There’s other scenarios. I’ll look into it and get some code pointers when I can.
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