Just keep it simple 🙂
grep + echo should suffice:
grep -qxF 'include "/configs/projectname.conf"' foo.bar || echo 'include "/configs/projectname.conf"' >> foo.bar
-q
be quiet-x
match the whole line-F
pattern is a plain string- https://linux.die.net/man/1/grep
Edit:
incorporated @cerin and @thijs-wouters suggestions.