pre-commit

# If you want to allow filename that differ only by case set this variable to true.
allowcasecollision=$(git config hooks.allowcasecollision)

if [ "$allowcasecollision" != "true" ] &&
	test "$(git diff --cached --name-only | 
	  LC_ALL=C tr [A-Z] [a-z] | 
	  sort | 
	  uniq --repeated )"
then
	echo "Error: Attempt to add a file name which differs only by case from "
	echo "another tracked file"
	echo
	echo "This can cause problems if you want to work"
	echo "with people on other platforms."
	echo
	echo "To be portable it is advisable to rename the file ..."
	echo
	echo "If you know what you are doing you can disable this"
	echo "check using:"
	echo
	echo "  git config hooks.allowcasecollision true"
	echo
	exit 1
fi

You could print to stderr instead, like this:

cat >&2 << EOF
Error: Attempt to add a file name which differs only by case from
another tracked file

This can cause problems if you want to work
with people on other platforms.

To be portable it is advisable to rename the file ...

If you know what you are doing you can disable this
check using:

  git config hooks.allowcasecollision true

EOF