deduplicate group messages across connections

Author: datwelk

src/SignalR/server/Core/src/DefaultHubLifetimeManager.cs

@@ -225,8 +225,7 @@ public override Task SendGroupAsync(string groupName, string methodName, object?
     public override Task SendGroupsAsync(IReadOnlyList<string> groupNames, string methodName, object?[] args, CancellationToken cancellationToken = default)
     {
         // Each task represents the list of tasks for each of the writes within a group
-        List<Task>? tasks = null;
-        SerializedHubMessage? message = null;
+        var connections = new ConcurrentDictionary<string, HubConnectionContext>();
 
         foreach (var groupName in groupNames)
         {
@@ -238,13 +237,16 @@ public override Task SendGroupsAsync(IReadOnlyList<string> groupNames, string me
             var group = _groups[groupName];
             if (group != null)
             {
-                DefaultHubLifetimeManager<THub>.SendToGroupConnections(methodName, args, group, null, null, ref tasks, ref message, cancellationToken);
+                foreach (var connection in group)
+                {
+                    connections.TryAdd(connection.Key, connection.Value);
+                }
             }
         }
 
-        if (tasks != null)
+        foreach (var connection in connections)
         {
-            return Task.WhenAll(tasks);
+            SendConnectionAsync(connection.Key, methodName, args, cancellationToken);
         }
 
         return Task.CompletedTask;