When executing the installer with arguments or using the unattend.xml to run it without UI as documented in https://python.readthedocs.io/en/stable/using/windows.html#installing-without-ui the installer ignores the arguments or unattend.xml when it's executed again.

The second time, the installer (running i modify mode) uses the default values (hereby removing the Prepenpath if set the first time)

(testet with version 3.6.6-amd64 to 3.7.3-amd64 of the executable installer)

To recreate, run the following : 

step 1 :
python-3.6.6-amd64.exe /passive InstallAllUsers=1 PrependPath=1 Include_doc=0 Include_dev=0 Include_tcltk=0 Include_test=0 SimpleInstall=1 

now python is install as expected.

step 2 : 
python-3.6.6-amd64.exe /passive InstallAllUsers=1 PrependPath=1 Include_doc=0 Include_dev=0 Include_tcltk=0 Include_test=0 SimpleInstall=1

even though the options are exactly the same, the installer now removes python from path and installs tcltk, documentation, test and dev.

step 3 : 
python-3.6.6-amd64.exe /passive InstallAllUsers=1 PrependPath=1 Include_doc=0 Include_dev=0 Include_tcltk=0 Include_test=0 SimpleInstall=1

now the installer only makes a quick check, what i would have expected in step 2, but hare only the default values are used.