Can be eyebrows be transplanted for the second time?

Yes you can have multiple transplants, but scarring happens with each transplant and that can reduce graft survivability. This can be mitigated by growth factors such as Acell or Amniofix.

Yes, eyebrow transplantation can be completed again to enhance the density following a previous procedure.  Since eyebrows are like a billboard on your face, it is critical to have natural looking eyebrows that are appealing to you and others.  If you feel that the density is not adequate being 12 months out, then it would be appropriate to have more placed, especially two follicle grafts in the middle of each one.

In my practice, I typically utilize single & 2 hair grafts when  restoring eyebrows. The thickness in the center of the eyebrow is improved in my patients by utilizing two hair grafts with single hair grafts in the upper & lower margins angled laterally (middle face outward) & directed toward the central double hair grafts. The central grafts are positioned preserving a gentle angled arc of lashes over the orbit of the eye.  This will give the best result regarding density.  I usually flare the eyelashes closest to the bridge of the nose at a 45 degree angle and offer additional density over the complete eyelash to allow the patient to pluck to the desired shape. This is often different among patients, males versus females. How thick & what shape one prefers is a personal decision. 

Dear Klaraaa, 
You can do a second eyebrows transplantation, but if some of the transplant hairs direction is wrong they need to be extract with very fine micro punchs. And after then a total new planning and very artistic ans natural eyebrows transplant will be possible 

If the eyebrow transplant did not grow, it can be retransplanted a second time without any problems

The great thing about FUE is that you can add as needed. I would recommend that you seek an in person consultation with a trusted hair transplant specialist that performs eyebrow transplants and get some options. Another transplant is definitely possible, whether that is a revision or an addition.