how to remove repeat element in a list

Hi 

I want to remove repeated element that in a list, but I can't find the related function for this.

I try to use hash table to solve it , but the order of this list will be changed

does any other function for this ??

ex:

                                                      change to 

aa= list("AA" "AA" "AA" "BB" "BB")  -----------------> aa = list("AA" "BB)