[General] How should I address strangers as in the American society if I don't know their names

[renard2]

And as I said before, repeating the person's name is also unnatural.

Agreed. Say it once, then leave it. If being polite is a concern, one could use modals (could, would, should) rather than addressing over and over.

For example:
"Could you tell me that again?"

Rather than:

"Sir, can you tell me that again?"

 

[arihant9n]

Thanks everyone for your valuable inputs! Renard2, yeah using modals would convey the same politeness as "Sir" does.