If I am not happy with the hair transplant result, can I have second one immediately?

It is normal to have asymmetries on your hair line, but if it's really disturbing you, you can get a touch up if it's not damaging the other grafts

Most people who have never had a hair transplant have uneven hairlines and it is natural.  Too much symmetry of a hairline can look quite unnatural-- just look at Senator Chuck Schumer's hairline.  The human eye is drawn to lines, hairlines are not suppose to be lines.
With that said in your specific -- if the you are unhappy with the uneven hairline, you can do another hair transplant now and do not need to wait until the transplanted hair comes out.