previously this would simply clone the shared pointer, but that leaves
references into the old tree behind which can then cause state confusion
(e.g. the asserts to do with m_parent consistency can fail). instead opt
for a deep-clone behavior. it's more expensive but also much much safer.
code is very exhaustive because we don't want to stack exhaust on
duplication so a recursive call won't do. instead we iterate down the
tree and then append all folders in reverse order.
add a test to ensure duplication works correctly
BUG: 463442