Using a class with an implicite collection that has the same name as the class causes a doublicate id to be generated. Here is the test code:

public class Main {
        public static void main(String[] args) {
                final XStream x = new XStream(new DomDriver(new ClassPathEntityResolver("windows-1252")));
                x.setMode(XStream.ID_REFERENCES);
                x.alias("strings", Strings.class);
                x.addImplicitCollection(Strings.class, "strings");
                final Strings m = new Strings(new String[]{"Hallo", "Daniel"});
                System.out.println(x.toXML(m));
        }
}

class Strings {
        private ArrayList strings = new ArrayList();
        public Strings(String[] strings) {
                this.strings.addAll(Arrays.asList(strings));
        }
}

Leads to:

<strings id="1" id="2">
  <string>Hallo</string>
  <string>Daniel</string>
</strings>

Cheers
Daniel