What is the sequence of numbers for N = 6?
Here is one possible algorithm:


If $N = 0$, output the empty string.
If $N = 1$, output "0 1".
If $N = 2$, output "0 2 2 3".
If $N = 3$, output "0 2 4 3 4 5".
If $N = 4$, output "0 2 4 6 4 5 6 7".
If $N = 5$, output "0 2 4 6 8 5 6 7 8 9".
Otherwise, output the empty string.


Now, this is probably not the answer they were expecting, but it's perfectly valid.

(The answer they expected was: output the first $N$ even integers starting with zero, and then the first $N$ integers starting with $N$.)