x = len(countries)
while n<x:
if countries[n]==countries[n+1]:
n +=1
if for example list lenght is 10 , last item in list is list[9] (and first is list[0])
if your countries lenght is 100, and last time when you are in a loop it tries "if counries[99] == counties[100]: (and there's error because last item in your list is countries[99] not [100] )
my brain's not working so well right now (been coding 10hours now), but hopefully that was the error in your code ;D