title: Prevent duplicate entries by column
body: <p>In Excel, is there a way (non VBA) to have a certain columns not accept entries if they are duplicated elsewhere in that particular column (not all columns, just column B for example)  </p>
  
  <p>In the below data, the second entry of EMP_ID 324 would not be allowed </p>
  
  <pre><code>Region     EMP_ID   Salary  Yrs_Experience
  Chicago     324    $165,140     10
  Chicago     234    $162,692     12
  NY           56    $122,451     10
  NY          567     $98,567     7
  SF          344    $116,735     8
  SF          324 
  </code></pre>