torrentListPropMap: deduplicate trackerDomains

Occasionally one torrent might have multiple tracker URLs from the
same domain. For instance, when there is separate ipv6 tracker URL
or when user use the "Set Tracker" feature.

Duplicate trackerDomains are not useful. It only causes the count
to be incremented multiple times and displayed incorrectly.

server/constants/torrentListPropMap.js

@@ -219,7 +219,8 @@ torrentListPropMap.set('trackerURIs', {
       }
     });
 
-    return trackerDomains;
+    // Deduplicate
+    return [...new Set(trackerDomains)];
   },
 });