/**
 * Returns error messages describing any duplicate `action_id` values within a
 * single block. Slack rejects a payload where two interactive elements in the
 * same block share an `action_id`. JSON Schema can't express per-property
 * uniqueness across sibling items, so this check sits alongside structural
 * validation like {@link findDuplicateBlockIds}.
 *
 * Uniqueness is required *within* a block, not across the payload: `state.values`
 * is keyed `block_id` then `action_id`, so the same `action_id` in two different
 * blocks is legal and is not flagged.
 * @param blocks - array of Block Kit blocks (or anything with a nested shape)
 * @returns array of human-readable error messages (empty when no duplicates)
 */
export declare function findDuplicateActionIds(blocks: readonly unknown[]): string[];
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