Don't force-add a tab list entry if it already exists

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -211,8 +211,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(PlayerListItem packet) {
-    serverConn.getPlayer().getTabList().processBackendPacket(packet);
-    return false; //Forward packet to player
+    return !serverConn.getPlayer().getTabList().processBackendPacket(packet);
   }
 
   @Override

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabList.java

@@ -129,8 +129,9 @@ public class VelocityTabList implements TabList {
    * Processes a tab list entry packet from the backend.
    *
    * @param packet the packet to process
+   * @return {@code true} to forward the packet on, {@code false} otherwise
    */
-  public void processBackendPacket(PlayerListItem packet) {
+  public boolean processBackendPacket(PlayerListItem packet) {
     // Packets are already forwarded on, so no need to do that here
     for (PlayerListItem.Item item : packet.getItems()) {
       UUID uuid = item.getUuid();
@@ -149,14 +150,13 @@ public class VelocityTabList implements TabList {
           if (name == null || properties == null) {
             throw new IllegalStateException("Got null game profile for ADD_PLAYER");
           }
-          entries.put(item.getUuid(), (VelocityTabListEntry) TabListEntry.builder()
+          return entries.putIfAbsent(item.getUuid(), (VelocityTabListEntry) TabListEntry.builder()
               .tabList(this)
               .profile(new GameProfile(uuid, name, properties))
               .displayName(item.getDisplayName())
               .latency(item.getLatency())
               .gameMode(item.getGameMode())
-              .build());
-          break;
+              .build()) == null;
         }
         case PlayerListItem.REMOVE_PLAYER:
           entries.remove(uuid);
@@ -187,6 +187,7 @@ public class VelocityTabList implements TabList {
           break;
       }
     }
+    return true;
   }
 
   void updateEntry(int action, TabListEntry entry) {

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabListLegacy.java

@@ -72,7 +72,7 @@ public class VelocityTabListLegacy extends VelocityTabList {
   }
 
   @Override
-  public void processBackendPacket(PlayerListItem packet) {
+  public boolean processBackendPacket(PlayerListItem packet) {
     Item item = packet.getItems().get(0); // Only one item per packet in 1.7
 
     switch (packet.getAction()) {
@@ -103,6 +103,7 @@ public class VelocityTabListLegacy extends VelocityTabList {
         break;
     }
 
+    return true;
   }
 
   @Override