I don't know of any quick and easy way to do that but if
they're small lists it shouldn't be hard. (If there is one I'd like
to know as well...) The best strategy would probably depend on the
size of the lists.
If they're not large, just a simple for each loop, adding
elements that don't already exist to one of the lists would
probably be easiest.
If they're very big, you may try searching out a
hashing/HashMap type ActionScript class from the internet to use..
or write your own. I don't think that data structure exists in AS
yet. But there's bound to be some class already written if you look
around online for one.
I know that in Java (and you'd have to confirm that the
hashing algorithm you use behaves similarly), when you add a value
to a HashMap with a key that already exists, it either overwrites
the old or doesn't add the new item. (I forget which it does.) This
effectively eliminates duplicates and since it's a hashing
algorithm it's pretty quick for larger data sets. After that's
done, if you can just get a list (as a Java HashMap allows) of all
the values in the hash map, your lists are combined and duplicates