1. create a temp history file:
cat > /tmp/history <<EOF
foo
foo
bar
other
bar
EOF

2. HISTFILE=/tmp/history emacs -Q

3. (setq comint-input-ignoredups t)

4. M-x shell

The expected history (ring-elements comint-input-ring) is ("bar"
"other" "bar" "foo"), but the actual result is ("bar" "other" "foo"
"foo").


The reason is comint-read-input-ring uses 0 as the index of the last
input. But the last input is actually inserted by
ring-insert-at-beginning, and it is the oldest item.