ninomerge

Hi I got two NI numbers one on my BRP and another one from a letter that I received from Department of Work and Pensions. My payslip has the NI number on my BRP and I'm taxed on an emergency tax code. I contacted the HMRC on phone about the wrong tax code and got informed that it could be because of the two NI numbers. I was told that I'll receive a letter but not yet received. How should I move forward?

I have got two National Insurance numbers. I would like to know if there is any way to merge these two.