When shard.yml contains duplicate keys for example two entries called dependencies, the spec parser concatenates the values:

yaml = <<-YAML
name: foo
version: 0.1.0

dependencies:
  foo:
    github: foo/foo
dependencies:
  bar:
    github: bar/bar
YAML
spec = Shards::Spec.from_yaml yaml

spec.dependencies.map(&.name) # => ["foo", "bar"]

This is unexpected and differes from any other YAML parser where previous values are simply overriden. For example YAML.parse behaves that way:

YAML.parse(yaml)["dependencies"].as_h.keys # => ["bar"]

As a result of this behaviour normalized YAML (for example passing yaml through YAML.parse(yaml).to_yaml parses differently than the ill-formed spec with duplicate keys.

I'd expect the parser to behave like other YAML parses and only use the last value. That's how it worked before #349. We could also consider to fails parsing such a a spec. Duplicate keys are clearly an error in the shard spec and should be fixed by the author. But issueing a warning is probably better.