RangeError for a half that is not an array, a name outside RENDERER_CAPABILITY_NAMES, or a name declared in both halves — a requirement already implies the interest an optional declaration states, so the duplicate is a contradiction in the caller's intent, refused rather than resolved by precedence.
Validates a RendererCapabilityDeclaration (§62, §85) — the one validation, run by RendererRegistry.resolve before any backend is constructed, and exported so an application layer forwarding a declaration (§45) can refuse it at its setup edge with the same rule.