CollapsibleIfStatements

Question

I recently stumpled upon the following warning using PMD (embedded in hudson), my code seems to suffer CollapsibleIfStatements, which I do not fully understand. The code looks like this

// list to be filled with unique Somethingness
List list = new ArrayList();

// fill list
for (SomeObject obj : getSomeObjects()) { // interating 
  if (!obj.getSomething().isEmpty()) { // check if "Something" is empty *
    if (!list.contains(obj.getSomething())) { // check if "Something" is already in my list **
      list.add(obj.getSomething()); // add "Something" to my list
    }
  }
}

In my opinion this code isn't more "collapsible" (otherwise it would be even more unreadable for the next guy reading the code). On the other hand I want solve this warning (without deactivating PMD ;).