If the law is worth its money, than you can start working in this position, wait for the trial period to end (or until they let you go) and then sue for the compensation stated in the job ad (well, in a country with some level of work place protection you could).

Or play their game, sign the contract, keep your original job, not show up and later tell them why they did waste their time themselves. If they then threaten you, you can point at the law.