If you need to know the number of unique values in the list here's a handy function:
Get_unique_values = Function( {in_list},
{Default Local},
tmp = [=> 0];
Insert Into( tmp, in_list );
tmp << get keys;
);
To use it, first compile the function, and then call it like this:
my_list = {"aa", "bb", "bb", "cc", "cc"};
print(get_unique_values(my_list);
{"aa", "bb", "cc"}