Reusing YAML anchor results in configuration error

[cowboy]

The problem

Per the YAML 1.2 spec, "anchors need not be unique" https://yaml.org/spec/1.2/#id2765878

However, the attached section in my configuration.yaml file reuses the turn_on anchor, and results in the following error when running ha core check:

Processing... Done.

Error: Testing configuration at /config
ERROR:homeassistant.util.yaml.loader:found duplicate anchor 'turn_on'; first occurrence
  in "/config/configuration.yaml", line 50, column 18
second occurrence
  in "/config/configuration.yaml", line 57, column 18
Failed config
  General Errors: 
    - Error loading /config/configuration.yaml: found duplicate anchor 'turn_on'; first occurrence
  in "/config/configuration.yaml", line 50, column 18
second occurrence
  in "/config/configuration.yaml", line 57, column 18

Successful config (partial)

I can work around this issue by giving the anchors unique names, eg. turn_on1, turn_on2, etc, but it seems like it should just work.

What is version of Home Assistant Core has the issue?

core-2021.8.8

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant OS

Integration causing the issue

No response

Link to integration documentation on our website

No response

Example YAML snippet

switch:
  - platform: template
    switches:
      one:
        value_template: "off"
        turn_on: &turn_on
          service: scene.turn_on
          target:
            entity_id: scene.one
        turn_off: *turn_on
      two:
        value_template: "off"
        turn_on: &turn_on
          service: scene.turn_on
          target:
            entity_id: scene.two
        turn_off: *turn_on

Anything in the logs that might be useful for us?

No response

Additional information

No response

[probot-home-assistant]

config documentation
config source
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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this issue as it has been labeled with an integration (config) you are listed as a code owner for? Thanks!
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[balloob]

We use YAML 1.1

[cowboy]

@balloob per the YAML 1.1 spec, "anchors need not be unique" https://yaml.org/spec/1.1/#id863390. Do you need me to open another issue, or will you reopen this one?

[balloob]

In that case it is an upstream issue in the YAML parser pyyaml

[cowboy]

In case it helps anyone else, this pyyaml PR fixes the issue: yaml/pyyaml#394. Hopefully they'll merge it.