Duplicate keys are not handled properly

YAML spec in version 1.2 says that in a valid YAML file, mapping keys are unique.

This is not the case in pyyaml, as can be seen by loading this sample file:

a:
  - b
  - c

a:
  q: a
  q: b

The correct result from loading this file is an error. pyyaml instead naively overwrites results with the last key, resulting in this dict: {'a': {'q': 'b'}}.

[alexchandel]

PyYAML should reject duplicate keys per the YAML spec. These are silent errors in users’ code. Do what ruamel.yaml does, and add a yaml.allow_duplicate_keys = True property for those who want to opt-in.

[perlpunk]

The YAML 1.1 spec also says that keys should be unique: https://yaml.org/spec/1.1/#id861060

The content of a mapping node is an unordered set of key: value node pairs, with the restriction that each of the keys is unique