en.json and de.json each defined the checks object twice; JSON silently
kept the second block's values, so the first block's friendlier titles/hints
(e.g. "Permission Mode" over the raw "--permission-mode auto support" flag)
were dead text. Merged into one block per key, keeping the friendlier title
and the more actionable hint.
Added a Utf8JsonReader-based duplicate-top-level-key test, since
JsonDocument accepts duplicates without error and the existing parity test
can't see this failure mode either.
