You mentioned writing a compareObjects method. Actually, you should override the equals method to return true when two objects are equal.
Having said that, I would just return a new list that contains unique elements from the original:
ArrayList original = ...
List uniques = new ArrayList();
for (T element : original) {
if (!uniques.contains(element)) {
uniques.add(element);
}
}
This only works if you override equals. See this question for more information.
