I found out that live donor can donate part of his liver only once.
There are some risks for the liver donor, just like with any major surgery: infection, different complications of surgery or anesthesia, pain, and in extreme cases, death. That is why donors are carefully screened for health issues. The biggest problem is that any major surgery is usually done for the benefit of the person undergoing surgery. In case of transplant, donor doesn't benefit is physical sence, but faces some risks. There are also psychological issues that donor may experience. Donor has to really be sure that he/she wants to go through with donation.
Fortunately, absolute majority of liver donors recover quickly after the procedure and live normal lives. The liver grows back in a few months in a donor as well as in a recipient.
I am not sure about the guidelines for second donation from the same living donor. If the first time went fine, and there are no health issues, I see no problem doing it again. I am doing research right now in the transplant department in the major university. I can ask my mentor about second time living donor transplantation. I will see him on Tuesday.
Hope this helps.