Consider the following document. Are *a and *b equal nodes? Does the map contain duplicate keys?

- &a [*a]
- &b [*b]
-
  *a: 1
  *b: 2

From https://yaml.org/spec/1.2/spec.html#id2764652:

Two sequences are equal only when they have the same tag and length, and each node in one sequence is equal to the corresponding node in the other sequence.

The nodes have the same tag and length, so it would seem that *a == *b if and only if *a == *b.