I received a 2019 1099-R, code P. Should I amend my 2018 return? How do I know I did the right things while I filed my 2018 return?

I overcontributed to my 401k in 2018 and the excess was returned to me before the tax deadline in 2019. TT forced me to mail my return. I think I reported the excess. I am trying to figure out if I paid the tax already or if by amending, i will pay it again. Thank you!