If the company makes up the previous unsigned period when signing the contract, employees can't ask for double wages. The term of signing a labor contract is generally one year.

People's Republic of China (PRC) labor contract law

Article 82 If an employer fails to conclude a written labor contract with the employee for more than one month but less than one year from the date of employment, it shall pay the employee twice the monthly salary.

Where an employing unit violates the provisions of this Law and fails to conclude an open-ended labor contract with its employees, it shall pay the employees twice the monthly salary from the date when the open-ended labor contract should be concluded.