ETL Validator does not run the same test case or test plan if it is already running in another instance by some user. At any given point, only one instance of the run will be active. As a result, all other users see a message saying " Another Instance of the selected test case is running" when they try to run the same test case. When you see this message, do NOT attempt to run such test case or test plan. Wait for sufficient time to allow ETL Validator Server to complete the execution.