fix(frontmatter): improve duplicate frontmatter key error handling

[codeafridi]

Summary

I fixed an issue with YAML frontmatter where duplicate keys caused a vague error that didn’t point to the real problem. I added a simple check that detects repeated keys and shows a clear message with the key name and line number which points to the file

Test plan

-ensured normal frontmatter files still parse correctly.
-ran the formatter manually to confirm it now identifies the duplicated key.
-verified that the error message displays the correct key and the line where it occurs.

Checklist

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• [ x ] I have read the contribution guidelines.

[martinjagodic]

This correctly prints to the console which key is duplicated on which line. It's frustrating that it doesn't tell me which file is affected in the collection view, so I have to guess that. It also breaks loading of affected entries - I get an empty card in the collection view if it's affected. I'm testing this on a large collection and my console is bobmarded with these messages

• Don't throw an error - this will enable the title to be displayed on the collection view even if an entry is affected. Show a warning instead, don't include a stack trace.
• Add info about the affected file along with the key and line number
• When an entry is open in Editor mode, only show errors for that file, not every file in the collection.

To discuss:
Should we display a warning to the user? If they are not a developer, they will have no control over this, so it could be annoying. However, this would be an effective way to fund such issues.

[codeafridi]

Thanks for the feedback. ill update it now.

• i will remove the thrown error and replace it with a simple console.log warn without a stack trace.
• i will include the file path, key name, and line number in the warning message so it’s clear which entry is affected.
  -then limit the detection to the currently opened entry, so warnings don’t appear for every file in the collection.
• yes cms users shouldn’t see this so i will keep it in the console only for developers.

I’ll push the changes soon.

[martinjagodic]

This is better now. For me, all files return as unknown while testing locally with decap proxy.

Warnings in the console are ok now. I noticed another problem: there are false positives when the same key appears on several levels. For example

title: root title
object:
  title: object title

will give an error that title is a duplicate on line 3, even though it should be fine.

Also, make sure that all tests are passing.

[codeafridi]

Alright now I get it. I'll now

-parse yaml structure properly
-track full key paths
-only warn when the same key appears in the same parent scope
-ennsuring all existing tests pass before pushing the update

[codeafridi]

do tell if any changes are needed.

[martinjagodic]

To make duplicate-key detection more robust and precise, please leverage the yaml parser’s built-in diagnostics instead of manual checks.

Concretely:

• Use yaml.parseDocument(src, { uniqueKeys: true, prettyErrors: true }).
• Inspect doc.errors/doc.warnings

This way, we can also pretty print any other formatting problems, not just duplicate keys.

Note: we had js-yaml and yaml both used in this project, but since #7684 we only have yaml.