Readd the clone detection.

src/main/java/com/imaginarycode/minecraft/redisbungee/RedisBungee.java

@@ -302,6 +302,23 @@ public final class RedisBungee extends Plugin implements Listener {
         }
     }
 
+    @EventHandler
+    public void onPreLogin(PreLoginEvent event) {
+        if (pool != null) {
+            Jedis rsc = pool.getResource();
+            try {
+                for (String server : configuration.getLinkedServers()) {
+                    if (rsc.sismember("server:" + server + ":usersOnline", event.getConnection().getName())) {
+                        event.setCancelled(true);
+                        event.setCancelReason("You are already logged on to this server.");
+                    }
+                }
+            } finally {
+                pool.returnResource(rsc);
+            }
+        }
+    }
+
     @EventHandler
     public void onPlayerConnect(final PostLoginEvent event) {
         if (pool != null) {