Arduino Due functioning but heating too much

I have recently purchased arduino due. The board is logically functioning fine (All Digital IOs are working in both input and output mode, serial communication on USB is also working fine) However the controller IC is heating too much within few seconds of applying power from USB. Temperature is rising approx. to 70DegC (158 DegF)
I have purchased this board from authorized distributor of aruduino (his website link is available on this page Distributors — Arduino Official Store )
I have already filed a support request and asked for RMA details.
To that he replied as due work on 3.3V logic and i might have applied 5V logic
I have not done so and all digital IOs are working fine. Upon this clarification I got second response and as per his reply this is perfectly normal behavior and I should continue using the same board.
I am sure this is not normal. so what should i do