Hey guys you all are giving the wrong answers. You may not have understood the question clearly. I know the answer so I am clarifying the question. You should have all digits consisting of 1 to 9 without repitition. You cannot take only some (say 13458 ). The number should contain nine digits consisting 1 to 9 and the double of it also should have 9 digits between 1 to 9 without repitition in both.

so you cannot say 6729 and its double is 13458 or similar which has less than nine digits.

The answer given by debonair is right.

123456789 and its double 246913578 each with 9 digits and no repitition