Make sure that you have done the following steps.
• Install the correct board support package for LinkIt Smart 7688 Duo and drivers.
• From Arduino IDE - > Tools -> Port is set the correct COMX (LinkIt Smart 7688 Duo ).
• If you have updated the firmware in the board and installed new board support package for LinkIt Smart 7688 Duo, restart Arduino IDE.
• Sometimes the COM Port gets blocked and ports won’t get displayed under Arduino IDE -> Tools. Then try to restart your PC.