Correct answer: A. Relative value of other voters' votes decreases
Explanation: Every voter’s ballot should be counted once and should have the same weight as every other valid ballot. If one person’s vote is accidentally counted twice, that voter receives an unintended extra influence. The error does not make all votes stronger or improve the voter list. Instead, it adds weight to one choice and makes the choices of other voters relatively less influential in the total result.
Thus, option A is correct. The phrase “relative value” is important: the other votes may still be physically counted, but their share of influence becomes smaller compared with the duplicated vote. Double counting can even change the winner in a close election, so accurate verification and counting are essential. Options B, C, and D describe outcomes opposite to the democratic requirement of equal voting power.