my ans
1. no penalty
2. you can use your annual leave to offset the notice period if the company agrees
3. if you are serving your notice in your old company, you cannot go to the new one.
Answers
1. No, but normally you will be blacklisted.
2. Company will continue to pay you for the notice period. You can use your annual leave to offset your notice period but subject to your supervisor's approval. (If you can not clear all you AL before you leave, they should encash your leave.)
3. If you use your leave offset the notice period and got approved. You can start in the new company early (It is legislative illegal but usually nobody will care about this.) Salary payed by both company will be taxable (You have to sum up the figures in IR8A form both company.)