title: If item is both in column A and B print in C
body: <p>Hello I have a really simple question. I have searched, and find lots of answers that compare matching cells in the same row or involve multiple rows, but nothing matching my really simple need.</p>
  <p>I want to compare two entire columns. If a unique item appears anywhere in both column A and also in Column B, then compile a new list in column C. We are trying to give rewards to users who are participating in two different programs, but there are hundreds of IDs in both lists and the IDs are a couple dozen characters each, so sorting this manually is not practical.</p>