I believe there are essentially two ways to avoid getting multiple answers to a question. One would be to disable *back buttons in the program so participants can't give multiple answers.
Unfortunately, at the moment GuidedTtrack does not have a feature to show only the latest answers in the CSV. I believe the easiest solution for you now would be to add some processing to the results CSV to extract the last answer.
Here is how this could be done in R with a regular expression:
x <- "3 | 4 | 2 "
sub(".*\\|([^\\|]*)$", "\\1", x)
Tiny demo: https://ideone.com/TbYOrc