[Avocado] Error if multiple default tags

This should be an error, because of duplicate default tags (`tag` properties inside a `yaml` block):

```
'''yaml
tag: package-2025-01-01-preview
'''

'''yaml
tag: package-2024-05-01-preview
'''
```

It eventually causes LintDiff to fail with a confusing error message:

> "message": "!Error: There are multiple operations defined for 'get: /subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/...'. You are probably trying to use an input with multiple API versions with an autorest V2 generator, and that will not work.",

This should print a clear error like "multiple default tags" in every tool that tries to parse readme.

Example: https://github.com/Azure/azure-rest-api-specs-pr/pull/21875/files#diff-ef1ae1eb921685889c04ccc56166f1f368c75d61acc71bac90370f95241b540cR23

https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4666452&view=logs&j=0574a2a6-2d0a-5ec6-40e4-4c6e2f70bea2&s=1cd41c50-172c-5e62-8939-616b494fad19&t=80c3e782-49f0-5d1c-70dd-cbee57bdd0c7&l=17345