Were the rotors and pads both replaced, or the rotors lathed last time the pads were replaced? What vehicle, and which (front, rear, disc or drum?) ?
Please clarify your question. Did you measure the rotors with a dial micrometer and find them thinner than the listed minimum? Or take them to be lathed and was told they're too thin?
I'm doubtful. They'd have to be removed, and why would you then put them back on?
Otherwise, where is this coming from, your idea that the rotors are thin? Are they or are they not working properly and without excessive grooves?
If they are working properly without excessive grooves there is no reason to replace them. If they are not, you must assess the whole brake system to determine if the rotors are at fault or not.
If there are deep grooves to the point where the pads are also unevenly worn, yes then you should replace both the rotors and pads.
How about linking some 45' angle pictures of the front and back of the worst rotor and pad?