What is wrong with duplicate keys?

Duplicate keys are allowed in regular YAML - as parsed by pyyaml, ruamel.yaml and poyo:

x: cow
y: dog
x: bull

Not only is it unclear whether x should be "cow" or "bull" (the parser will decide 'bull', but did you know that?), if there are 200 lines between x: cow and x: bull, a user might very likely change the first x and erroneously believe that the resulting value of x has been changed - when it hasn't.

In order to avoid all possible confusion, StrictYAML will simply refuse to parse this and will only accept associative arrays where all of the keys are unique. It will throw a DuplicateKeysDisallowed exception.