I looked at the script. It appears to be a simple race in case of simultaneous logins. Changing
echo "" > $stamp /usr/lib/update-notifier/apt-check --human-readable >> $stamp echo "" >> $stamp
to
{ echo ""; /usr/lib/update-notifier/apt-check --human-readable; echo ""; } > $stamp
might be enough to make it go away.
I looked at the script. It appears to be a simple race in case of simultaneous logins. Changing
echo "" > $stamp lib/update- notifier/ apt-check --human-readable >> $stamp
/usr/
echo "" >> $stamp
to
{ echo ""; /usr/lib/ update- notifier/ apt-check --human-readable; echo ""; } > $stamp
might be enough to make it go away.