Notice that the number 123456789 is a 9-digit number consisting exactly
the numbers from 1 to 9, with no duplication. Double it we will obtain
246913578, which happens to be another 9-digit number consisting exactly
the numbers from 1 to 9, only in a different permutation. Check to see
the result if we double it again!
Now you are suppose to check if there are more numbers with this
property. That is, double a given number with k digits, you are to tell
if the resulting number consists of only a permutation of the digits in
the original number.