Prevent duplicate entries in a list.

src/main/java/org/qortal/list/ResourceList.java

@@ -99,8 +99,10 @@ public class ResourceList {
     /* List management */
 
     public void add(String resource) {
+        if (!this.contains(resource)) {
             this.list.add(resource);
         }
+    }
 
     public void remove(String resource) {
         this.list.remove(resource);