First sale: 50 kilograms of rice are sold to the first purchaser and duly quantified for that purchaser.
Second sale: the same rice is sold anew as 50 kilograms. The second buyer needs a fresh valid measurement.
If the new result is 49 kilograms, a shortage question arises; if it is 51, the entitlement to the excess must be resolved. The point is reliable ascertainment of what is sold, not simply repeating a mechanical ritual.