Duplicate << keys shouldn't be marked as error when merge=true

[ikatyang]

version: 1.0.0-rc.7

var YAML = require("yaml").default;

YAML.parse(`
.anchors:
  - &anchor1
    key: value
  - &anchor2
    another: prop
foo:
  bar: baz
  <<: *anchor1
  <<: *anchor2
`, { merge: true }) //=> YAMLSemanticError: Map keys must be unique; "<<" is repeated

Expected: no error

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Original post: prettier/prettier#4919