Sorting through an array

I have an array of song artists and songs indexed by an id, i need a way of taking out all the artists names. However as there are multiple lines for each artist i have come accross a problem when only displaying the artist once. Another problem is that the artists are not grouped together so i need to store the artists that i have already passed and havent been matched so i can check the next artist. I know it sounds confusing, but i hope you understand what i mean! Any help would be appreciated. Jamie