title: How to tell if a cell exists on another Google Sheet
body: <p>I have 12 sheets in one Google Sheets document labeled for each month (January - December). On each sheet column A contains a project number, e.g. "6091". </p>
  
  <p>I'm trying to find a function that will check all of the other sheets to see if there are duplicate cells in the "project number" column of other sheets.</p>
  
  <p>So: "Do any of the cells in column A, match any of the cells in column A on other sheets".</p>
  
  <p>Is there a quick way to do this?</p>