guarded against duplicate targets

as suggested by @tloist

Author: FND

lib/index.js

@@ -11,14 +11,16 @@ module.exports = {
 };
 
 function faucetJS(config, assetManager, { browsers, compact, sourcemaps } = {}) {
+	let targets = new Set();
 	let bundlers = config.map(bundleConfig => makeBundler(bundleConfig,
-			assetManager, { browsers, compact, sourcemaps }));
+			assetManager, { browsers, compact, sourcemaps, targets }));
 
 	return filepaths => Promise.all(bundlers.
 		map(bundler => bundler(filepaths)));
 }
 
-function makeBundler(bundleConfig, assetManager, { browsers, compact, sourcemaps } = {}) {
+function makeBundler(bundleConfig, assetManager,
+		{ browsers, compact, sourcemaps, targets }) {
 	// NB: bundle-specific configuration can override global options
 	let config = Object.assign({ sourcemaps, compact }, bundleConfig, { browsers });
 	if(!compact) {
@@ -35,6 +37,10 @@ function makeBundler(bundleConfig, assetManager, { browsers, compact, sourcemaps
 	let { resolvePath } = assetManager;
 	entryPoint = resolvePath(entryPoint);
 	target = resolvePath(target, { enforceRelative: true });
+	if(targets.has(target)) {
+		throw new Error(`duplicate target: ${target}`);
+	}
+	targets.add(target);
 	let bundle = new Bundle(entryPoint, target, config, { browsers });
 
 	let writer = makeWriter(bundle, assetManager);