The following simple module can be used to generate the more complex vectors in my comment above.

start rep(a, b, c, d, inc);
return ( j(1, d, 1) @ do(b, c, inc) @ j(1, a, 1) );
finish;

For example:

x = rep(2, 1, 3, 1, 1); gives 1,1,2,2,3,3
x = rep(1, 3, 1, 2, -1); gives 3,2,1,3,2,1
x = rep(2, 0, 6, 1, 2); gives 0,0,2,2,4,4,6,6

repetition factors are also possible at both ends, so:

x = rep(2, 0, 12, 3, 4);

gives.

0,0,4,4,8,8,12,12,0,0,4,4,8,8,12,12,0,0,4,4,8,8,12,12