Just print:
1)2*n-1
2)1 2 ... n
3)n n-1 ... 2

It's work, but why? example
4
1 2 3 4 4 3 2
original 4->3->2->1->2->3->4
They don't meet

They meet at the 6th point(3 = 3).

Why do you need to print that 1 at the beginning?

You don't need it. 2 3 4 4 3 2 also works fine. But 3 2 1 3 2 (or 2 3 4 2 3) is shorter.