 If you liked
Kämpfer
|
 ...then you might like
Nurarihyon no Mago
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Both series involve the main male protagonist having 2 appearances while having the same memory (For Mago it's a while before he retains his memory of his other self).
And.... the male guy has a crush on a girl who likes his other self, and the girl thinks that the other self is his friend.
Differences: Mago is youkai-related while Kampfer focuses on ecchi and fighting between 2 sides: red and blue.