My theory on how the story is going to tackle the "twin" concept is either sisters with very distinct personalities or a distant relative of one of the current girlfriends who happen to be almost physically identical.
I feel like twins would be too normal. In this story I think the “twin” archetype would have to be something like 10 sisters although that would take up a lot of the slots in his harem.
When you have 100 characters I don't think it's exactly a sell out to make two of the characters identical twins, especially since whilst it would seemingly fill up two slots with the same person, it's really worth it for the power of twins, twins are just so amazing and you can do so many things with them.
First you can have twins say the same sentence in sync, or have them finish each other's sentences, you can also have them wear identical clothes or swap clothes and be like "who's who? If you get it right we'll give you a reward 😉" and I think it's really cute when they hold both of their hands and look sideways at the MC, especially if they also have their cheeks pressing together, and then there's always the classic good twin evil twin scenario, where one twin acts really modest and sweet and well mannered, while the other one is rude and naughty...
don't. You. Think. Every. Manga. Should. Have. Twins?