Add method to check if an entry already exists in the tab list.

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

@@ -57,6 +57,8 @@ public class VelocityTabList implements TabList {
 
   @Override
   public Optional<TabListEntry> removeEntry(UUID uuid) {
+    Preconditions.checkNotNull(uuid, "uuid");
+
     TabListEntry entry = entries.remove(uuid);
     if (entry != null) {
       PlayerListItem.Item packetItem = PlayerListItem.Item.from(entry);
@@ -67,6 +69,12 @@ public class VelocityTabList implements TabList {
     return Optional.ofNullable(entry);
   }
 
+  @Override
+  public boolean containsEntry(UUID uuid) {
+    Preconditions.checkNotNull(uuid, "uuid");
+    return entries.containsKey(uuid);
+  }
+
   /**
    * Clears all entries from the tab list. Note that the entries are written with
    * {@link MinecraftConnection#delayedWrite(Object)}, so make sure to do an explicit