An attribute cannot have two arguments with the same canonical name:
{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}
{% 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” %}
{% 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” %}
{% 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 for more details on why FIDL requires declarations to have unique canonical names.
