 If you liked
Kuzu no Honkai
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 ...then you might like
Tatoe Todokanu Ito da toshitemo
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Both are based on the theme that the protagonist loves a person who already loves/is in relationship with someone else.
In Tatoe Todokanu Ito da toshitemo: a girl loves her childhood friend who married her brother.
In Kuzu no Honkai: a girl loves her childhood friend who became a teacher at the school she attends and fell in love with another teacher (same sex relationship also appears here).