Duplicate key handling limits in YAML 1.2 parsers

Goal: Determine a consistent, spec‑compliant approach for YAML 1.2 parsers when a mapping contains duplicate keys, so that applications can rely on predictable data structures across different libraries.

Current YAML 1.2 specifications leave the handling of duplicate keys within a single mapping undefined, allowing implementations to raise an error, overwrite the earlier value, or collect both. This variability creates portability problems for documents processed by multiple parsers, especially in configurations or data‑exchange scenarios where duplicate keys may appear unintentionally.

- What behavior should a conforming YAML 1.2 parser exhibit for duplicate keys within a single mapping? - Should duplicate keys be treated as a parse error, cause the later value to overwrite the earlier one, or result in a collection of all values? - How can applications detect and mitigate inconsistent duplicate‑key handling when switching between YAML processors?