If you are applying via non-SDS, an English certificate is not required (although, having one might improve your profile in the eyes of the VO). However, if you want to submit a certificate with your application, you should use one that's accepted by IRCC and that's the IELTS. There is another one that I think is also accepted by IRCC but I can't remember the name right now.
However, if you want apply for a University or College and they accept the Duolingo test, then go for it and then decide if you want to also take the IELTS to submit it for your Study Permit application.