If a class has methods with the same name, one is annotated with jdk.jpackage.test.Annotations.Test annotation. Others will also be executed as tests. E.g.:

class Foo {
    @jdk.jpackage.test.Annotations.Test
    @jdk.jpackage.test.Annotations.Parameter("1")
    public void a(int v) {}

    public void a() {} // jpackage test-lib will consider this method as a test, but it shouldn't
}

Another, more likely, way this error occurs is when a parameterized test function has the same name as a function supplying parameters:

class Foo {
    @Test
    @ParameterSupplier("testDates")
    public void testDates(LocalDate v) {
    }

    // jpackage test-lib will consider this method as a test, but it shouldn't
    public static Collection<Object[]> testDates() {
        return List.of(new Object[][] {
            { LocalDate.parse("2018-05-05") },
            { LocalDate.parse("2018-07-11") },
        });
    }
}

This error doesn't affect existing tests. It was discovered in not yet integrated tests.