I will generally use a hash, with the keys reflecting the unique data elements. As I load the hash, I'll increment the value each time I encounter the key -- that way I have a unique set of keys, yet I can know (if I bother to check) about duplicate values in my original data set. This technique is good for when I am handling data that I need to be unique but might contain duplicates -- I can produce a warning or take some other action when I encounter the duplicate key (yet my main process, using the resulting hash keys, is unhampered by duplicates).