I have obtained a waiver before for another article I published, do I need to request another waiver for the new article?

Yes. Generally universities/institutions have set their policies so that waivers apply at an article level. Faculty members cannot permanently opt themselves out of this arrangement, but they can opt out on an article-by-article basis. This means that, for each article you wish to publish on a subscription basis with IOP Publishing, you will need to request a waiver.