prevent race condition when processing players

src/main/java/org/zeroBzeroT/anarchyqueue/Queue.java

@@ -16,6 +16,7 @@
 import java.util.UUID;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Semaphore;
 
 // velocity api event docs:
 // https://jd.papermc.io/velocity/3.3.0/com/velocitypowered/api/event/package-summary.html
@@ -25,6 +26,8 @@ public class Queue {
 
     private final ProxyServer proxyServer;
 
+    private final Semaphore processSemaphore = new Semaphore(1);
+
     /**
      * We don't use ConcurrentLinkedQueue for this because we want index-based access to players.
      */
@@ -52,6 +55,8 @@ public void onServerConnectedEvent(ServerConnectedEvent e) {
     }
 
     public void process() {
+        if (!processSemaphore.tryAcquire()) return;
+
         // check queue server reachability
         final RegisteredServer serverQueue;
 
@@ -113,6 +118,8 @@ public void process() {
                             queuedPlayers.removeFirst();
                         }
                 );
+
+        processSemaphore.release();
     }
 
     private void sendInfo(RegisteredServer serverQueue, boolean full) {