Just sign with the new firm and tell the current company as soon as possible. Generally these employment contracts are not worth the legal hassle to actually try to enforce their rights, especially if you haven't started work at the firm.

s://.mom.gov.sg/faq/contract-of-service/if-a-new-employee-has-signed-a-letter-of-employment-but-does-not-show-up-for-work-what-can-an-employer-do