Loops in a dataframe- R

I have a dataframe that looks like this (but for every US county)

Countyname	Neighbour County	Neighbour State
Autauga County, AL	Chilton County	AL
Autauga County, AL	Dallas County	AL
Baldwin County, AL	Escambia County	FL
Catron County, NM	Apache County	AZ

For all counties, if the neighbouring county is in the same state I want to replace the value in the Neighbour State column with a missing value NA, and if it's in a different state I want to keep it unchanged. I.e. I want to end up with something like this:

Countyname	Neighbour County	Neighbour State
Autauga County, AL	Chilton County	NA
Autauga County, AL	Dallas County	NA
Baldwin County, AL	Escambia County	FL
Catron County, NM	Apache County	AZ

I was thinking of looping through each row and if the column 'Countyname' contains the entry in the column 'Neighbour State', replace the entry with NA (e.g. for the first row if 'Autauga County, AL' contains 'AL', replace 'AL' with NA). How would I go about this (or is there a more efficient way to do it as this feels clunky)?