projectlombok/lombok · error · IllegalArgumentException

Duplicate argument not allowed

Error message

Duplicate argument not allowed: <key>

What it means

parseArgs rejects supplying the same non-'conf.' key twice. Only keys prefixed 'conf.' may repeat (values joined with the path separator); anything else throws IllegalArgumentException "Duplicate argument not allowed: <key>".

Solutions

1. Remove the duplicate occurrence of the named argument
2. If you intended multiple values, use the 'conf.' prefix keys which accumulate with the path separator
3. Deduplicate arguments programmatically in generating scripts
4. Use a map-style construction so each key appears once

Example fix

// before
... type=A type=B
// after
... type=A  (or conf.path=A conf.path=B for accumulating keys)

function parseArgs(argv) {
  const seen = new Set();
  for (const a of argv) {
    const key = a.substring(0, a.indexOf('=') === -1 ? a.length : a.indexOf('=')).replace(/^-+/, '');
    if (seen.has(key) && !key.startsWith('conf.')) throw new Error('Duplicate argument not allowed: ' + key);
    seen.add(key);
  }
}

try { parseArgs(argv); } catch (IllegalArgumentException e) { if (e.getMessage().startsWith("Duplicate argument")) { dedupeAndRerun(e.getMessage()); } throw e; }

When it happens

Trigger: Passing e.g. 'type=x type=y' or a duplicated flag on the command line; key does not start with 'conf.'.

Common situations: Shell scripts appending arguments that are already hardcoded, loop-generated argument lists emitting a key twice, copy-pasted flags.

Understand the failure class

Background: "Unknown argument", "Invalid value", and "must be one of": invalid CLI argument errors explained — this error's family across 35 libraries.

Related errors

• mandatory argument ' ' missing
• 4 args required: [path to creds file] [path to file root to…
• 4th arg must be 'true' or 'false'
• 4th argument must be one of 'all', 'changelog'…
• Format keys need to be 2 values separated with a colon.

AI-assisted analysis of projectlombok/lombok@6d6a3e9fec (2026-09-07). Data as JSON: /api/errors/07cbc7871f510144. Report an issue: GitHub.

Appendix: source

Thrown at src/support/lombok/eclipseCreate/CreateEclipseDebugTarget.java:189

			"   shadowLoaderBased                       # Add the VM options to use lombok as an agent and pass the classpath to the shadow loader. Needed for ECJ/Eclipse.\n" +
			"   conf.test=foo:bar:baz                   # Where 'test' is an ivy conf name, and 'foo' is a path to a jar, relativized vs. current directory.\n" +
			"   favorite                                # Should the debug target be marked as favourite?\n" +
			"");
	}
	
	private static Map<String, String> parseArgs(String[] args) throws IllegalArgumentException {
		Map<String, String> map = new LinkedHashMap<String, String>();
		
		for (String arg : args) {
			int idx = arg.indexOf('=');
			String key = (idx == -1 ? arg : arg.substring(0, idx)).trim();
			String value = (idx == -1 ? "" : arg.substring(idx + 1)).trim();
			String existing = map.get(key);
			if (existing != null) {
				if (key.startsWith("conf.")) {
					value = existing + File.pathSeparator + value;
				} else {
					throw new IllegalArgumentException("Duplicate argument not allowed: " + key);
				}
			}
			map.put(key, value);
		}
		return map;
	}
}

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