It's a given same face would be an issue with story-driven content :p! Mainly because it can become disorienting for the reader, especially if there aren't additional markers to separate characters (like colors, maybe people can get away with tones)
I think another thing that can bring up same face is also just not understanding how facial anatomy works - and when you don't know how something works, how can you break the rules?
Anyway, I always fret with whether or not I'm having issues with same face. I know I have a tendency to draw noses in a certain way because I like the way it looks when it's drawn that way. Most of the time I'm also lazy with drawing the minute differences when I really shouldn't be.,,, whoops
another test for fun is something someone from tumblr made up, the 'Babs and Kara Test' - which is essentially just drawing your characters in bathrobes with their hair wrapped up in a towel and seeing if they're different enough to be identifiable