Include duplicate assets in the manifest

Editor assessment

`feat` is correct — surfacing duplicate assets is new manifest behavior with API implications — but the header lacks a scope; `feat(build):` or `feat(manifest):` would direct the entry to the right changelog section and make filters like `git log --grep` more precise. The subject itself is clear, imperative, and properly formatted, mentioning both the what (duplicate assets) and the where (the manifest). Because consumers programmatically read manifests, a body or docs update describing the new shape matters; without it, downstream adopters must reverse-engineer the format from the diff.