Upon entering a code R 2018 Form 1099-R into 2018 TurboTax would simply be reminding you that this should have been reported on your 2017 tax return. (2018 TurboTax's bullet point should say "If you've already filed your 2017 tax return without entering this recharacterization, you'll need to amend it.") Since you already did that, you can ignore the code R 2018 Form 1099-R.
