Use case or problem
I’m not sure if this qualifies as a bug per se. Perhaps it’s intentional. But if I type a quotation mark after a letter, only a single mark appears, which is the correct behaviour. However, after another punctuation mark such as a period, typing a quotation mark produces a pair, requiring me to delete the second quotation mark. This becomes more annoying when using the Smart Typography plugin, because I have to type a backspace and then delete in order to get the “right” quotation mark in place.
Proposed solution
I would suggest that typing a quotation mark after punctuation should behave the same way as when typing after a letter – a single mark. I can’t think of a case where this would be inconvenient.