Shichinin no Nana

In both animes theres sisters/copys who pretty much look alike, each one with a different personality. 

While they have their differences, for example, Osomatsu-San is aimed at a more mature audience, (If you're looking for something a little more risque, read the Shichinin no Nana manga. (o_o)) and Shichinin no Nana is somewhat is aiming for a consistant story in the end. They both have the theme of "multiple of the same face living under a roof." (except Shichinin no Nana has one 1 more). All have their own personalities, and they conflict with each other almost every episode. It's also worth noting that each Nana and Matsuno has a unique color to help them stand out. Nanacchi and Jyushimatsu, for example, are both yellow because they are hyperactive and always happy.  read more 

Both these titles are about the multiplying power of the microwave oven. In one series, the microwave is used to create multiple timelines. In the other series, the microwave is used to create multiple Nanas. I recommend these series to microwave fans, and all others. 

Sgt. Frog's artist, Mine Yoshizaki, created the character designs for Seven of Seven. there is various crossovers exist between Sgt. Frog and Nana 7 of 7. Also, in episode 89 of Sgt. Frog, it features a plot that splits Corporal Giroro into seven personalities, most of whom resemble Nana's split personalities