Question

Is it okay to say Mr. sir, if not why not

Answer 1

Answer: Unless the person you're referring to has “sir” in their name, you shouldn't refer to them as Mr. Sir, because you can't use the formal greetings together.

Answer 2

It is improper grammar to say "Mr.sir". It is redundant. "Mr." already introduces the person as a man, so to say "sir" would be improper.

Hope this helps!