72 yrs old completed RMD form 4852 for 2015 return since didn't rec. funds or 1099. Now have a1099R for 2015 (received March 2016) and 2016 RMD amounts. Am I paying taxes twice on that 2015 amount?

I can't tell if I paid taxes on 2015 return or not on that RMD amount. Don't understand why fill out form 4852 if not being taxed on that RMD distribution in 2015.