Firstly, there is no Employment Law which determines when an employer can recruit again. It may be that your employer now has a need to fill the job that you were made redundant from.
You don't specify the type of job that you were doing so I will use an example. If you were a 'sales representative' looking after an account with a company that stopped using your services, it may be that your job no longer existed which led to your redundancy. If say two months later your 'previous' employer won the contract back which required another 'sales representative' then are entitled to recruit for this position again.
Have you contacted your old employer about this? You may wish to be considered for the role again? if you do, I would advise that you contact the company and enquire about applying for your old job.
If you are unhappy about the job being re advertised, I would advise you to write a formal grievance to your employer (send a copy to the HR department and your Line Manager). The company has 4 weeks to reply to your grievance even though you no longer work there. Keep a copy of this letter.
If you believe that your redundancy was unfair for any reason, you have up to 6 months from the end of your employment to challenge the decision in an employment tribunal (you don't say in your question whether you appealed the redundancy? There are many reasons which fall into the category of ‘unfair reason for selection’ if you are selected for redundancy for any of these reasons, you will be found to have been unfairly dismissed if you were unfairly selected for redundancy including membership or non-membership of a trade union, because you work part-time, exercising your statutory rights, you are a fixed term worker etc.