Well ... isnt there a running joke that Penny might be Pizza Girl ?
And the Secret Bakery of Northampton has a character that looks exactly like Faye, except she's black.
More recently many robots got intoduced which have more or less humanoid looks. Except they are missing stuff, like a nose, drawn a bit simpler, etc. And they use skin colors that arent human, like blue or bright pink.
That sort of demonstrates the problem if you have a comic thats relatively "simplified" in character looks you have only a relatively limited amount of options to give people an own face, but at the same time need quite large amounts of characters, especially because constantly new ones are introduced.
Fortunategly large parts of our brain are into processing power for visuals, and large parts of that are into understanding faces specifically. So even small differences will be picked up.
