Data validation - Drop down list with no duplicates in excel
<p>Could you please assist me  with <strong>removing duplicates</strong> in a <strong>drop down list</strong>. The list is not static. Example:</p>

<p><strong>Before</strong></p>

<pre><code>James
Peter
James
Nick
Peter
</code></pre>

<p><strong>After</strong></p>

<pre><code>James
Peter
Nick
</code></pre>

<p>I am not sure what formula to use data validation formula box.</p>

<p>I had attempted the below, but no success:</p>

<pre><code>=OFFSET($C$13,0,0,COUNTIF(C:C,"?*")-1)
</code></pre>