SQL & PL/SQL

I have a string variable - varchar2(240) maximum - which is arbitarily filled with 3 letter codes. Any one or more of these codes may be duplicated within the string. What I need to do is to strip out all the duplications and end up with a single contiguous string containing all the unique code options. e.g.

Initial string 'AAABBBCCCBBBDDDAAAEEEBBB'

should end up as :-

'AAABBBCCCDDDEEE'

Hope this makes sense - also there is no need to place the unique codes in any kind of order.

Thanks for your help...