def WebIDLTest(parser, harness):
    parser.parse(
        """
        interface Foo;
        interface Bar;
        interface Foo;
        """
    )

    results = parser.finish()

    # There should be no duplicate interfaces in the result.
    expectedNames = sorted(["Foo", "Bar"])
    actualNames = sorted(map(lambda iface: iface.identifier.name, results))
    harness.check(
        actualNames, expectedNames, "Parser shouldn't output duplicate names."
    )