added deduplication of cluster node list

Deduplication happens for both cluster and force_cluster because
clustering with the same node twice doesn't make sense in either case.

Author: Alexandru Scvortov

src/rabbit_mnesia.erl

@@ -284,7 +284,7 @@ delete_cluster_nodes_config() ->
 %% specified cluster nodes.  If Force is false, don't allow
 %% connections to offline nodes.
 init_db(ClusterNodes, Force) ->
-    ProperClusterNodes = ClusterNodes -- [node()],
+    ProperClusterNodes = lists:usort(ClusterNodes -- [node()]),
     case mnesia:change_config(extra_db_nodes, ProperClusterNodes) of
         {ok, Nodes} ->
             case Force of