 If you liked
Mirai Nikki (TV)
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 ...then you might like
Nazo no Kanojo X
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This recommendation may be a stretch, but these two series both gave me weird feelings while watching them. Mysterious Girlfriend X is more light-hearted and lovey-dovey while Mirai Nikki has more of a serious tone to the whole anime. Both series have their main characters obsessed over one another (Yuno and Yukki in Mirai Nikki, Tsubaki and Urabe in MGX). If you liked MGX, I think you should be able to like Mirai Nikki, and vice versa.
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Recommended by SirLanceALot
Both have the Crazy/Weird Girlfriend aspect. Both have the mysterious girlfriend aspect as well.
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Recommended by BlazeAnime