yellow code highlight: indicates which lines are considered “new”
red marginal marker: indicates lines not covered by tests. You may also see green (covered) and a yellow “barber pole” (partially covered)
grey/brown marginal marker: indicates a duplicated block. You can click this marker to get some detail on the duplication. Click it again to get links to the the other location(s) of the block.
From your comment, I believe you’re interpreting the yellow code overlay as a duplication indicator, which is not correct.