The diff adds a second top-level dsv4-fp4-mi355x-sglang-agentic-hicache: mapping key immediately below the pre-existing one, right before the new dsv4-fp4-mi355x-sglang-agentic-mtp: entry that this PR is actually meant to introduce. In the resulting file this shows up as two adjacent, byte-for-byte identical blocks: the pre-existing entry at lines 2452-2466, followed by the newly-added duplicate at lines 2468-2482.

Code path: configs/amd-master.yaml is loaded via yaml.safe_load in load_config_files() (utils/matrix_logic/validation.py:919). PyYAML's default loader does not error on duplicate mapping keys within a single document — it silently keeps only the last occurrence. Since both dsv4-fp4-mi355x-sglang-agentic-hicache blocks are identical, the "last one wins" collapse produces the exact same effective config as if the duplicate had never been added, so there is no behavioral difference today.

Why the repo's own duplicate-key guard doesn't catch it: load_config_files() does have a duplicate-key check at validation.py:930-936, but it only detects a key being duplicated across different config files (it intersects config_data.keys() from the current file against all_config_data.keys() accumulated from previously-loaded files). By the time that check runs, yaml.safe_load has already silently collapsed the intra-file duplicate down to one key, so the duplication within amd-master.yaml itself is invisible to it. There's also no .yamllint config in the repo that would otherwise flag duplicate keys at CI time.

Impact: Purely cosmetic/dead config today, since the two blocks are identical and the parser deterministically resolves to one value. However, this is exactly the kind of artifact that causes real confusion later: if a future PR edits one of the two blocks (e.g. bumping the image tag or tweaking the search space) without noticing the sibling duplicate, the edit could silently have no effect (if the untouched copy happens to load last) or silently take effect while looking like a no-op in a diff review. It's clearly unintentional — the PR description says it's adding only the new -mtp entry, and the mechanism is obviously that the hicache block was copied as a scaffold for the new entry and the copy of the original block was never deleted.

Proof / walkthrough:

1. Before this PR: dsv4-fp4-mi355x-sglang-agentic-hicache: appears once, at (pre-PR) line ~2465.
2. This PR's diff inserts 32 new lines directly after it, starting with dsv4-fp4-mi355x-sglang-agentic-hicache: again (identical image, model, scenarios, search-space), followed by the new dsv4-fp4-mi355x-sglang-agentic-mtp: block.
3. Post-PR, grep -n '^dsv4-fp4-mi355x-sglang-agentic-hicache:' configs/amd-master.yaml returns two hits (lines 2452 and 2468 in the final file).
4. Running the file through yaml.safe_load collapses these to a single key with the (identical) value of the second occurrence — no functional change today, but a landmine for future edits and a source of reviewer confusion.

Fix: Delete the duplicated dsv4-fp4-mi355x-sglang-agentic-hicache: block introduced by this PR (lines 2468-2482), leaving the original entry and the new dsv4-fp4-mi355x-sglang-agentic-mtp: entry intact.