Disable re-adminning infinitely (#11174)

Content.Server/Administration/Managers/AdminManager.cs

@@ -87,6 +87,11 @@ namespace Content.Server.Administration.Managers
                 throw new ArgumentException($"Player {session} is not an admin");
             }
 
+            if (reg.Data.Active)
+            {
+                return;
+            }
+
             _chat.DispatchServerMessage(session, Loc.GetString("admin-manager-became-admin-message"));
 
             var plyData = session.ContentData()!;