Avoid race condition when adding servers

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.cs

@@ -1222,10 +1222,14 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
                         serverSnapshot = new ServerEndPoint[configuration.EndPoints.Count];
                         foreach (var endpoint in configuration.EndPoints)
                         {
-                            var server = new ServerEndPoint(this, endpoint, log);
-                            serverSnapshot[index++] = server;
+                            var server = (ServerEndPoint)servers[endpoint];
+                            if (server == null)
+                            {
+                                server = new ServerEndPoint(this, endpoint, log);
                                 servers.Add(endpoint, server);
                             }
+                            serverSnapshot[index++] = server;
+                        }
                     }
                     foreach (var server in serverSnapshot)
                     {