avajs/ava · error · Error

Unexpected duplicate extensions in options: ’${[...duplicate

Error message

Unexpected duplicate extensions in options: ’${[...duplicates].join('’, ’')}’.

What it means

AVA builds the final extension list from your `extensions` option plus extensions contributed by providers (like @ava/typescript). If the same extension ends up registered more than once, resolveExtensions throws to keep the extension pattern unambiguous.

Source

Thrown at lib/extensions.js:29

				seen.add(ext);
			}
		}
	};

	if (configuredExtensions !== undefined) {
		if (!Array.isArray(configuredExtensions)) {
			throw new TypeError('The extensions option must be an array');
		}

		combine(configuredExtensions);
	}

	for (const {main} of providers) {
		combine(main.extensions);
	}

	if (duplicates.size > 0) {
		throw new Error(`Unexpected duplicate extensions in options: ’${[...duplicates].join('’, ’')}’.`);
	}

	// Unless the default was used by providers, as long as the extensions aren't explicitly set, set the default.
	if (configuredExtensions === undefined) {
		if (!seen.has('mjs')) {
			seen.add('mjs');
		}

		if (!seen.has('js')) {
			seen.add('js');
		}
	}

	return [...seen];
}

View on GitHub (pinned to bbfd946322)

Solutions

1. Remove duplicate entries from the `extensions` array.
2. If @ava/typescript is installed, drop 'ts' from your list and let the provider supply it.
3. Deduplicate programmatically when building config: `[...new Set(extensions)]`.
4. Check config merge logic (CLI flags + config file) for accidental concatenation.

Example fix

// before
export default {extensions: ['js', 'mjs', 'mjs']};
// after
export default {extensions: ['js', 'mjs']};

const exts = config.extensions ?? [];
const dups = exts.filter((e, i) => exts.indexOf(e) !== i);
if (dups.length) throw new Error(`Duplicate extensions: ${dups}`);

const hasNoDuplicateExtensions = v => !Array.isArray(v) || new Set(v).size === v.length;

try {
  await run();
} catch (err) {
  if (err.message.startsWith('Unexpected duplicate extensions')) {
    console.error('Deduplicate the extensions option; provider extensions (e.g. @ava/typescript) count too');
    process.exitCode = 1;
  } else throw err;
}

When it happens

Trigger: Listing the same extension twice, e.g. `extensions: ['js', 'mjs', 'js']`, or combining your `extensions` config with a provider (e.g. @ava/typescript) that already contributes 'ts' and re-adding 'ts' yourself.

Common situations: Merging config objects where both base config and override add the same extension; adding 'ts' manually while @ava/typescript is installed; accumulating extensions across config layers without deduplication.

Related errors

• The ’environmentVariables’ configuration must be an object c
• The extensions option must be an array
• The ’files’ configuration must be an array containing glob p
• The ’watchMode.ignoreChanges’ configuration must be an array
• ${fileForErrorMessage} must export a plain object or factory

AI-assisted analysis of avajs/ava@bbfd946322 (2026-09-02). Data as JSON: /api/errors/c5443828bbc1986f. Report an issue: GitHub.