Double tap S for BnS

[Molyapina]

Hi

Im currently using kind of script in BnS
$F::
While, GetKeyState("f","p")
{
Send {f}
Sleep 1
Send {4}
Sleep 1
}
Return
f11::
suspend
return

It works well but now i wish to add or have another script for backstep ability. When double tapping S character jumps back. I've tried with that same script and other several ones that i found out but cant get it to working. Anyone with any advices or even ready script how to do it?