Fixed term emploment

R/Sir XYZ PSU recruited first batch Fixed term employees on March 2014 for a specific project on fixed term contract for a period of 3 years. After 10 months XYZ PSU again recruited second batch fixed term employees on Jan 2015 for the same project on fixed term contract.The process of recruitment was same for both the cases and eligibility criteria was also same. The pay and perks given to the employees in the above two cases are different with the latter employee enjoying far better pay and perks than the former employee. Second batch is getting HRA,yearly increment and other allowances while first batch is getting no such allowances. Company's HR department is saying that First batch cannot get such allowances as it is not mentioned in their contract. Both the employees are doing same work in same condition with same qualifications still the latter case is getting benefitted . Former case employees are suffering just because they joined 10 months earlier. Having served 10 months more still they are getting meager pay and perks than their junior batch employees. Can the first batch employee claim pay parity with the second and junior batch employees?? Waiting for your advice!