If someone scratches my car door and then later a second person knocks it off completely and the second person has to compensate, is the first person who scratched the door still responsible to pay me?
(Based on a true story.)
I seem to recall that you're paid for the difference between the market value of the car before it was damaged and its value damaged: that the payment has nothing to do with repairs. (This differs from assault on a person, where one pays for the difference in value (as above, nezek), repairs (ripuy), lost wages (sheves), and pain and suffering (boshes and tzaar).) That would, if true, imply (a) that the second person would not, as the question assumes, need to replace the door; (b) that the second person would pay only the difference between the value of a scratched car and that of a doorless car, which would mean the owner is not getting paid for the scratch at all if not from the first person; and (c) that the first person would need to pay the difference between the value of an unscratched car and that of a scratched car. But I have no source.
After the first person scratches it, he becomes obligated to pay for the damage. The nizak (damagee) wouldn't profit though, since the second person would only need to pay to replace the scratched door, not a whole new door. If the only way to replace it is to put on a new door, I assume the second person would be compensated for the improvement.