Does Air France actually weigh carry ons? And where?

First time flying international and first time with Air France, so trying to avoid surprises.

I keep hearing that AF weighs carry on bags. Mine is definitely overweight. Do they usually check this at check in, or is it more of a gate thing? I’m debating whether to just pay for it ahead of time and be done with it.

Also flying from the US with a layover in CDG, then onward to Nigeria. If I end up paying for extra weight, is that a one time thing for the whole journey, or do they hit you again in Paris?

Trying to figure this out before I get burned at the airport.