As an F pax travelling to both Uganda and Ethiopia, i have had my carry-on weighed at the gate several times recently. I only ever take one backpack which holds laptop, hard drive and all those spare lithium ion batteries that can't go in the cargo hold. The bag usually weighs in at 8 to 9 kg. Apart from a stern look, I was not castigated or fined and allowed to board.
The silly thing is, I was laden down with a large amount of heavy duty free and this was completely ignored. It would easily have weighed 20kg. Duty free seems to be invisible most of the time and yet it is often considerably heavier than carry-on baggage.
It makes me wonder whether weight is really such an issue when so much additional weight is not quantified and allowed on board. Perhaps the weighing process is indeed a cash grab. Perhaps also, DDF doesn't want pax to consider weight when splashing their cash in duty free. It might significantly curtail sales.