throw error on duplicated import

src/Module.js

@@ -5,6 +5,7 @@ import MagicString from 'magic-string';
 import analyse from './ast/analyse';
 import { has } from './utils/object';
 import { sequence } from './utils/promise';
+import getLocation from './utils/getLocation';
 
 const emptyArrayPromise = Promise.resolve([]);
 
@@ -76,6 +77,13 @@ export default class Module {
 					const localName = specifier.local.name;
 					const name = isDefault ? 'default' : isNamespace ? '*' : specifier.imported.name;
 
+					if ( has( this.imports, localName ) ) {
+						const err = new Error( `Duplicated import '${localName}'` );
+						err.file = this.path;
+						err.loc = getLocation( this.code.original, specifier.start );
+						throw err;
+					}
+
 					this.imports[ localName ] = {
 						source,
 						name,

src/utils/getLocation.js

@@ -0,0 +1,20 @@
+export default function getLocation ( source, charIndex ) {
+	const lines = source.split( '\n' );
+	const len = lines.length;
+
+	let lineStart = 0;
+	let i;
+
+	for ( i = 0; i < len; i += 1 ) {
+		const line = lines[i];
+		const lineEnd =  lineStart + line.length + 1; // +1 for newline
+
+		if ( lineEnd > charIndex ) {
+			return { line: i + 1, column: charIndex - lineStart };
+		}
+
+		lineStart = lineEnd;
+	}
+
+	throw new Error( 'Could not determine location of character' );
+}

test/samples/duplicate-import-specifier-fails/_config.js

@@ -1,9 +1,12 @@
+var path = require( 'path' );
 var assert = require( 'assert' );
 
 module.exports = {
 	description: 'disallows duplicate import specifiers',
 	error: function ( err ) {
-		assert.ok( false ); // TK - pick an error message
+		assert.equal( err.file, path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { line: 1, column: 12 });
+		assert.ok( /Duplicated import/.test( err.message ) );
 	}
 };
 

test/samples/duplicate-import-specifier-fails/main.js

@@ -1,2 +1,2 @@
-import { a, a } from './exporter';
+import { a, a } from './foo';
 assert.equal(a, 1);