PathDB keys diff layers by state root, not by block hash. That means a
side-chain block can legitimately collide with an existing canonical diff layer
when both blocks produce the same post-state (for example same parent,
same coinbase, no txs).
Today `layerTree.add` blindly inserts that second layer. If the root
already exists, this overwrites `tree.layers[root]` and appends the same
root to the mutation lookup again. Later account/storage lookups resolve
that root to the wrong diff layer, which can corrupt reads for descendant
canonical states.
At runtime, the corruption is silent: no error is logged and no invariant check
fires. State reads against affected descendants simply return stale data
from the wrong diff layer (for example, an account balance that reflects one
fewer block reward), which can propagate into RPC responses and block
validation.
This change makes duplicate-root inserts idempotent. A second layer with
the same state root does not add any new retrievable state to a tree that is
already keyed by root; keeping the original layer preserves the existing parent
chain and avoids polluting the lookup history with duplicate roots.
The regression test imports a canonical chain of two layers followed by
a fork layer at height 1 with the same state root but a different block hash.
Before the fix, account and storage lookups at the head resolve the fork
layer instead of the canonical one. After the fix, the duplicate insert is
skipped and lookups remain correct.
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