The "existing" /tmp come from casper code :
bionic/casper-1.394/scripts/casper-bottom/12fstab ... cat > $FSTAB <<EOF ${UNIONFS} / ${UNIONFS} rw 0 0 tmpfs /tmp tmpfs nosuid,nodev 0 0 EOF ...
The "existing" /tmp come from casper code :
bionic/ casper- 1.394/scripts/ casper- bottom/ 12fstab
...
cat > $FSTAB <<EOF
${UNIONFS} / ${UNIONFS} rw 0 0
tmpfs /tmp tmpfs nosuid,nodev 0 0
EOF
...