Have mirena (levonorgestrel) for a year, stopped periods after 6 months, now returned 2 nd month in a row as a light spotting. normal?

Yes: In normal periods the estrogen your ovary produces makes the uterine lining grow and after ovulation the progesterone tells it to stop growing and get mature. The Mirena (levonorgestrel) is giving your uterus constant exposure to a progestin in addition to what your ovaries are producing so the lining is constantly being told not to grow. This results in less lining and thus very light to nonexistent periods.