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

api/src/main/java/com/velocitypowered/api/proxy/player/TabList.java

@@ -37,12 +37,20 @@ public interface TabList {
    * Removes the {@link TabListEntry} from the tab list with the {@link GameProfile} identified with
    * the specified {@link UUID}.
    *
-   * @param uuid of the
+   * @param uuid of the entry
    * @return {@link Optional} containing the removed {@link TabListEntry} if present, otherwise
    * {@link Optional#empty()}
    */
   Optional<TabListEntry> removeEntry(UUID uuid);
 
+  /**
+   * Determines if the specified entry exists in the tab list.
+   *
+   * @param uuid the UUID of the entry
+   * @return {@code true} if it exists, {@code false} if it does not
+   */
+  boolean containsEntry(UUID uuid);
+
   /**
    * Returns an immutable {@link Collection} of the {@link TabListEntry}s 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