Add another test case.

test/function/relative-external-include-once-two-external/_config.js

@@ -0,0 +1,22 @@
+var assert = require( 'assert' );
+var path = require( 'path' );
+
+
+module.exports = {
+	description: 'includes a relative external module only once (two external deps)',
+	options: {
+		external: [
+			path.join( __dirname, './foo.js' ),
+			path.join( __dirname, './first/foo.js' )
+		]
+	},
+	context: {
+		require: function ( required ) {
+			assert( [ './foo.js', './first/foo.js' ].indexOf(required) !== -1, 'required wrong module' );
+			return required === './foo.js' ? 'a' : 'b';
+		}
+	},
+	exports: function ( exports ) {
+		assert( exports === 'ab' || exports === 'ba', 'two different modules should be required' );
+	}
+};

test/function/relative-external-include-once-two-external/first/module.js

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+import foo from './foo';
+
+export default foo;

test/function/relative-external-include-once-two-external/main.js

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+import foo from './foo';
+import first from './first/module';
+
+export default foo + first;