Updated, the update... should be working now hopefully...

RelayServer/Services/Chat/ConnectedClientService.cs

@@ -0,0 +1,56 @@
+using System.Collections.Concurrent;
+
+namespace RelayServer.Services.Chat;
+
+public static class ConnectedClientService
+{
+    private static readonly ConcurrentDictionary<string, string> SessionToUsername = new();
+    private static readonly ConcurrentDictionary<string, HashSet<string>> UsernameToSessions =
+        new(StringComparer.OrdinalIgnoreCase);
+
+    public static void Register(string sessionId, string username)
+    {
+        if (SessionToUsername.TryGetValue(sessionId, out var oldUsername) &&
+            !string.Equals(oldUsername, username, StringComparison.OrdinalIgnoreCase))
+        {
+            RemoveSessionFromUsername(sessionId, oldUsername);
+        }
+
+        SessionToUsername[sessionId] = username;
+
+        var sessions = UsernameToSessions.GetOrAdd(username, _ => new HashSet<string>(StringComparer.Ordinal));
+        lock (sessions)
+            sessions.Add(sessionId);
+    }
+
+    public static void Unregister(string sessionId)
+    {
+        if (SessionToUsername.TryRemove(sessionId, out var username))
+            RemoveSessionFromUsername(sessionId, username);
+    }
+
+    public static IReadOnlyCollection<string> GetSessionsForUser(string username)
+    {
+        if (UsernameToSessions.TryGetValue(username, out var sessions))
+            lock (sessions)
+                return sessions.ToList();
+
+        return Array.Empty<string>();
+    }
+
+    public static string? GetUsernameForSession(string sessionId) =>
+        SessionToUsername.TryGetValue(sessionId, out var u) ? u : null;
+
+    private static void RemoveSessionFromUsername(string sessionId, string username)
+    {
+        if (!UsernameToSessions.TryGetValue(username, out var sessions))
+            return;
+
+        lock (sessions)
+        {
+            sessions.Remove(sessionId);
+            if (sessions.Count == 0)
+                UsernameToSessions.TryRemove(username, out _);
+        }
+    }
+}