during the service new Contract agreement with employer

Hi every one .i am an employee of one construction company .i sign offer letter and start the job.after completion of 1.5 year they give men an other contract to sign by hook or crook.they black mail to sign .this contract they segregate my salary to avoid to pay over time and Saturday work during the stay in company.
my question is this it is legally right or wrong they can do or if we sign this agreement the 1st one null&wide or remain active.

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