How can I fill in every second block?

Regardless of what exactly you want to do with the slabs the correct answer was already given: the clone command will be your friend.

Basically it's /clone x1 y1 z1 x2 y2 z2 x3 y3 z3, where 1 and 2 are your boundaries of what you want to clone and 3 is the target area, which in your case can as well be ~ ~ ~, but we'll come to that later.

Let's say you want to copy some slabs like these:

1. Note the coordinates of the two outside stones. These will be the boundaries.
2. If you want to clone the stones upwards (East or South where the coords go up) you can simply do this:
   • stand next to the last slab (leave one space for air if you didn't copy one after the last slab)
   • Use the clone command: /clone x1 y1 z1 x2 y2 z2 ~ ~ ~
     (In my case: /clone -627 4 -341 -643 4 -341 ~ ~ ~)
3. If you need to clone to the other direction (West, North) it is important to subtract the length of the source area from the target coords:
   • clone will always go upwards, so you need to start further down
   • Let's say I have 143 blocks to clone (-643 to -501):
     /clone -501 4 -341 -643 4 -341 ~-143 ~ ~
4. Use the new boundary stone slab's coords and clone it again: /clone NEWx1 y1 NEWz1 x2 y2 z2 ~(-length) ~ ~(-length)

This way you can clone more and faster until you're happy with the result: