How to correctly check if adding file into sudoers.d would conflict with other files?

Question

I have made a shell script to write /etc/sudoers.d/aptget -file but it is breaking the sudo if an alias is already defined in /etc/sudoers or in any file in /etc/sudoers.d/. I am using visudo to edit the files but it does not catch the error as it seems to check only the file currently being edited.

Previously I added following manually into /etc/sudoers

Cmnd_Alias APTGETUPDATE = /usr/bin/apt-get update *
Cmnd_Alias APTGETINSTALL = /usr/bin/apt-get -y --force-yes install *

%aptgroup ALL = (root) NOPASSWD: APTGETUPDATE, APTGETINSTALL
Defaults!APTGETUPDATE !requiretty
Defaults!APTGETINSTALL !requiretty

Everything worked great. Making a script to write a into /etc/sudoers.d/aptget or adding above manually into it passes visudo check but causes sudo to not function.

script.sh:

#!/bin/bash

updaterSudoers="$(cat <<EOF
Cmnd_Alias APTGETUPDATE = /usr/bin/apt-get update *
Cmnd_Alias APTGETINSTALL = /usr/bin/apt-get -y --force-yes install *

%aptgroup ALL = (root) NOPASSWD: APTGETUPDATE, APTGETINSTALL
Defaults!APTGETUPDATE !requiretty
Defaults!APTGETINSTALL !requiretty
EOF
)"

echo "$updaterSudoers" | (EDITOR="tee -a" visudo -f /etc/sudoers.d/aptget)

error:

>>> /etc/sudoers: Alias `APTGETUPDATE' already defined near line 31 <<<
>>> /etc/sudoers: Alias `APTGETINSTALL' already defined near line 31 <<<
sudo: parse error in /etc/sudoers near line 31
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

Lines from 31 onwards in sudoers:

31  Cmnd_Alias APTGETUPDATE = /usr/bin/apt-get update *
32  Cmnd_Alias APTGETINSTALL = /usr/bin/apt-get -y --force-yes install *
33  
34  %aptgroup ALL = (root) NOPASSWD: APTGETUPDATE, APTGETINSTALL
35  Defaults!APTGETUPDATE !requiretty
36  Defaults!APTGETINSTALL !requiretty

Answer 1

Temporarily including the original sudoers file and clearing the old file seemd to do the trick.

includeCurrent=$'#include /etc/sudoers\n'
echo "$includeCurrent$updaterSudoers" | visudo -c -q -f -

Whole sudoers.sh I ended up having.

#!/bin/bash

#Clear the current file if it exsists 
#Warning: this might break stuff if you edit current user visudo rights
if [[ -f "/etc/sudoers.d/aptget" ]]
then
  echo -n > "/etc/sudoers.d/aptget"
fi

#New settings to be added
updaterSudoers="$(cat <<EOF
Cmnd_Alias APTGETUPDATE = /usr/bin/apt-get update *
Cmnd_Alias APTGETINSTALL = /usr/bin/apt-get -y --force-yes install *

%aptgroup ALL = (root) NOPASSWD: APTGETUPDATE, APTGETINSTALL
Defaults!APTGETUPDATE !requiretty
Defaults!APTGETINSTALL !requiretty
EOF
)"

#Include current sudoers file
includeCurrent=$'#include /etc/sudoers\n'

#Validate new settings with current settings
echo "$includeCurrent$updaterSudoers" | visudo -c -q -f -
if [ "$?" -eq 0 ]
then
  #Write /etc/sudoers.d/aptget with updateSudoers value
  echo "$updaterSudoers" | (EDITOR="tee" visudo -f /etc/sudoers.d/aptget) &> /dev/null
else
  #Yell and exit in case of error
  echo "ERROR CHECKING SUDOERS FILE:"
  echo "$includeCurrent$updaterSudoers" | visudo -c -f -
  exit 1
fi

#If sudoers.d or sudoers.d/aptget is not included include only sudoers.d/aptget
grep -xq "#includedir /etc/sudoers\.d/\?" /etc/sudoers || grep -xq "#include /etc/sudoers\.d/aptget" /etc/sudoers
if [ "$?" -eq 1 ]
then
  echo "#include /etc/sudoers.d/aptget" | (EDITOR="tee -a" visudo -f /etc/sudoers) &> /dev/null
fi