My reservice is in this coming August and there is a
compulsary IPPT test during reservice.
Can I take ippt earlier and then book for IPT and complete the 8
sessions before the reservice starts?
And if I complete the IPT before the reservice ippt, does that mean
that my current window is cleared even if i dont pass the reservice
ippt?

yes. you can book for IPT directly and once u completed the IPT,
your IPPT for current window will be consider clear, no matter you
pass or failed. But pls be reminded that the IPT have limited slots
only and usually need to be booked early.
And sometime, even if u clear IPT or IPPT before reservist, your CO
will still want you to participate in the reservist IPPT, just to
make up the number. This happen in my ICT, when we got a new CO.
All those who passed outside have to take the IPPT again, but then
the CO is not concern about the results, but more on the
participation.