You shouldn't follow a "whether" with an "or not." That's "the rule." But then, to ask "whether something has been done" is a difficult question to answer for somepeople; it "doesn't compute" in their brain that quickly, so we suggest using "if" instead.
It's especially dificult to answer when you start it with "May I know..."
"May I know if you have disabled the firewall?"
"But I still see evidence of the firewall in place!"
"Yes, that's because it is still enabled."
"But you told me it was disabled."
"No, I told you that you could know if it was disabled. And now you do."
The preferred question is a straight-up "Have you disabled the firewall?" followed by "Please disable the firewall."
- For Teachers