This can happen if you refresh the scan progress page. The browser is asking whether it should re-submit the original form, which sounds alarming but is safe. If a scan is already running (in this or another tab), the system will not start a second one. It will show a message that a scan is in progress and disable the button. You can safely click Cancel in the browser dialog to dismiss the warning, or click Continue, either way, the running scan is not affected.