Add select options only if option doesn't already exist

Add select options only if option doesn't already exist

Hi -

I have two multiple selects on my page.  One gets updated via ajax whenever the user filters data (we'll call that select "#select-1"), when they select options from that it then gets moved into the other select (we'll call that "#select-2").  That is all working fine but I'd like to refine it so that they can't select the same data twice. 

So, basically, what I'd like to do is add logic within the ajax call to only update the select options in (#select-1) if it does NOT exist in the other select (#select-2).

I have no clue where to even begin with this....do I need to store the val's in an array and then search for each one in the other? Does that even make sense?!  Help??