Add support for Duplicate keys for `check_yaml` hook

According to the YAML specification, keys must be unique:

3.2.1.1. Nodes: 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.

Although, unfortunately, some real-world YAML configurations may not be conformant with this point and duplicate keys may appear. A clear example of this is the Cirrus CI configuration file, which can define multiple task fields.

PyYAML enforces this option by default, which I totally agree with, but would be cool to specifically allow it. This is fairly simple to implement:

yaml = ruamel.yaml.YAML()
yaml.allow_duplicate_keys = True
yaml.load(stream)

https://yaml.readthedocs.io/en/latest/api.html#duplicate-keys

[asottile]

indeed, --unsafe is a blanket flag to disable all of the strict checking. if you’re going to violate the specification then all bets are off

[asottile]

also for cirrus you should use {uninquename}_task for naming your keys to conform with yaml specification