HDM should evaluate YAML data

[tuxmea]

• duplicate keys (on top level)
• duplicate keys (in hash)

[bastelfreak]

puppet-syntax is usually used in pipelines to validate data in modules: https://github.com/voxpupuli/puppet-syntax/blob/master/lib/puppet-syntax/hiera.rb

It currently has no check for duplicate keys, I think it should be added there.