Increasing density after 1st hair transplant?

Many patients go on to have a second hair transplant surgery to increase their hair density and address further hair loss. 

It is clearly possible to increase your density by adding a second procedure or pass through the area where you had your first transplant.  A second pass is often performed to increase coverage as hair loss progresses.  If the first pass has a thin density, you can increase density by adding a second pass in the same area.  Often times, a second or third pass is necessary in the crown or back of the head, while a dense packing alone suffices in the front of the scalp.  In a bald scalp, well performed hair transplantation actually improves circulation to the scalp.  A second pass is a great way to add more density following a properly performed hair transplant procedure.  
Of course, as in any field, there are poorly performed hair transplant procedures that can kill existing hair, leave patients with pitting, ridging, and a pluggy, unnatural appearance.  Procedures like this can poison the skin and make it difficult to get good growth in subsequent procedures.  In other words, do your home work first.  Don't depend on sites that recommend doctors for a fee.  Many of these sites recommend some poor physicians and relatively novice physicians, as well as some good physicians.  Really good surgeons don't require a someone claiming to be a hair transplant mentor, hair transplant patient advocate, or paid hair transplant consumer advocate to promote their good work.  If you do your homework well, you will not need to worry if the second pass will add density, because the second pass will add density.  If you take the advice from a paid advocate, you can expect good results, average results, lack luster results, or poor results.  Remember, these individuals are charging physicians $12,000 or more annually to recommend physicians.  Greed can get in the way of good judgement so proceed cautiously.  Advocates will also steer you away from the very best physicians, who do not pay their marketing fee.  There is something inherently wrong with that system and attempt to monopolize the industry through their refusal to deal.  
There is something else to consider if you add more hair.  Suppose you had no hair before your first procedure and you have 2000 grafts.  A year later, you will be 100% better.  However, if you add 2000 more grafts in the same area a year later, you will now have 4000 total grafts, but you will be only 50% better.