title: How to take two lists and combine them excluding any duplicates?
body: <p>I'd like to make one list from two separate lists of unique items.</p>
  
  <p>There are other similar questions but there didn't seem to be any that concerned doing this problem effectively since the lists are a few million items long.</p>
  
  <p>Totally unrelated: am I the only one who hates how the tags suggestion box covers up the "post your question" button?</p>