Is your linux configured to shut down on power button press and immediately shuts down after the next boot?
acpi_listen to check if you are getting two events for each button press:
button/power PBTN 00000080 00000000 button/power LNXPWRBN:00 00000080 0000001c
Hack around it by passing the event information to your script
... action=/etc/acpi/powerbtn.sh %e
And reacting only on one
... [ "$2" == "PBTN" ] || exit 0