Parser reports false positive error on yaml with anchors

[evidolob]

Describe the bug
When parser parses this yaml:

default: &DEFAULT
  name: Anchor
customname: &CUSTOMNAME
  custom_name: Anchor
anchor_test:
  <<: *DEFAULT
  <<: *CUSTOMNAME

It reports error: Map keys must be unique. But this is valid yaml.

To Reproduce
Just parse yaml provided in description.

Expected behaviour
Parser shouldn't report duplicate key errors on anchor references.

Versions (please complete the following information):

• Environment: Node.js 12.7.0
• yaml: 2.0.0-6

Additional context
Found during work on redhat-developer/yaml-language-server#442

[eemeli]

Your example is valid YAML 1.1, but we're defaulting to YAML 1.2, which does not consider << as any sort of special key.

To parse this as you expect, you may use either the merge: true or version: '1.1' options.

[evidolob]

yes, I totally miss that, thank you!