First, don't you think that Ami and Yumi look like twins? Well, my mom saw the commercial and said that it reminded her of us. (Yeah, we're twins and japanese)
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It really depends on what phase of their career you are seeing images of them from. Currently I think they are trying to emphasize their differences in looks and style as of late. This could be for the show, but also they are two different people growing older and less in the spotlight (and marketing machine that is Japanese culture) as they once were. They do have some similar looks, but when any amount of scrutiny is applied it is readily apparent they are not identical.
Over the course of their career Ami and Yumi's similarities have been played off more than now. I think once they got established as artists they did not need to have the sisterly look-a-like scheme going. It is no knock on them at all, many bands have used a similar thing...the Beatles come straight to mind.
Puffy is a great duo, no matter how they are packaged.