Never bought two of the same mod because I am always upgrading or looking at a new feature. I have a few that I would buy again if the price was right though. Unless you count ego's from back in the day. I have 4 ego twists floating around here somewhere. Probably junk by now because they haven't been charged in years.
Tanks are another story, I have multiples of a few. And some that I would never buy again. And some I gave the spares to other people. Back in the days of stardust clearomizers and cart tanks I always had several because they were small and pain to fill. So I would have like 5 tanks with different flavors I could just swap out.
If I know I am going to vape multiple flavors, like testing them or something I just use a RDA instead.