The Kabocha Wine: Nita no Aijou Monogatari (1984)

Nita, a dog that lives in Sunshine Academy's student dormitory, brings home with him a puppy that suspiciously looks like him—this sends everyone into an uproar as it is eventually concluded that Nita likely had a puppy with somebody else. Elle, Shunsuke, and the others collaborate with one another to take care of it, but a delinquent biker boy named Takizawa uses this opportunity to trick Elle into joining him on his yacht for nefarious purposes. Shunsuke & the others, having realized this, rush in to save Elle from Takizawa before it's too late.