Why am I charged twice in one month, that is, double payment?

The first SIM card did not work so had to be resent another. The site would not allow me to keep my first number as "it is already in use". Therefore I am registered under two numbers. As a consequence, I was charged twice this month: 03 Jan. and 20 Jan. How do I straighten this out so that I am paying under one number only and which number? I am currently using the second number.