Added a note on how to mark a bug as duplicate.

src/IDF/templates/faq.html

@@ -3,6 +3,7 @@
 {block body}
 <ul>
 <li><a href="#q-keyboard">What are the keyboard shortcuts?</a></li>
+<li><a href="#q-duplicate">How to mark an issue as duplicate?</a></li>
 </ul>
 
 <h2 id="q-keyboard">What are the keyboard shortcuts?</h2>
@@ -16,6 +17,15 @@
 <li><kbd>Shift+w</kbd>: The issues assigned to you.</li>
 </ul>
 
+<h2 id="q-duplicate">How to mark an issue as duplicate?</h2>
+
+<p>This is simple:</p>
+<ol>
+<li>Write in the comments "This is a duplicate of issue 123", change 123 with the corresponding issue number.</li>
+<li>Change the status of the current issue to <em>Duplicate</em>.</li>
+<li>Submit the changes.</li>
+</ol>
+ 
 {/block}
 {block context}
 <p>{trans 'Here we are, just to help you.'}</p>