docs/reference/fidl/language/error-catalog/_fi-0131.md

## fi-0131: Duplicate canonical attribute argument {:#fi-0131}

An attribute cannot have two arguments with the same canonical name:

{% include "docs/reference/fidl/language/error-catalog/label/_bad.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0131.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

Even though `custom_arg` and `CustomArg` look different, they are both
represented by the **canonical** name `custom_arg`. You get the canonical name
by converting the original name to `snake_case`.

To fix the error, give each argument a name that is unique after
canonicalization:

{% include "docs/reference/fidl/language/error-catalog/label/_good.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0131-a.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

Alternatively, remove one of the arguments:

{% include "docs/reference/fidl/language/error-catalog/label/_good.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0131-b.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

See [fi-0035](#fi-0035) for more details on why FIDL requires declarations to
have unique canonical names.