R/exams: Duplicated images in PDF files

When using several R/exams exercises with TikZ plots, I have no problems generating exams in HTML format or for Canvas. However, exactly the same exercises when rendered to PDF show the same plots in different questions. As a simple example, you can use the same exercise twice, e.g., for the automaton exercise shipped with the package:

library("exams")
set.seed(0)
exams2pdf(c("automaton.Rnw", "automaton.Rnw"))

The automaton diagram in the exercise should have a double circle for "A" (the "accepting state" described in the text). However, it has the double circle for "B" (as in the first exercise).