Uwagaki
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https://bookwalker.jp/series/393430/list/
It's fairly obvious to Terui Chiaki that her classmate Ajikawa Yoshio has a crush on her. He is completely not boyfriend material in her eyes and, even if he was, Chiaki already has a boyfriend. Their mysterious teacher Mr. Yamada sees this relationship between the two students and offers an experiment. He is able to make copies of people and objects and does so with Chiaki, creating a copy of her, except the copy has no knowledge of her boyfriend. Yoshio will hit on the Chiaki copy, who is named Koaki, and he'll try to make her fall in love with him. After a specified amount of time, the copy and original will be combined back together and which ever love is stronger between the current boyfriend and Yoshio will overwrite all other emotions. So this love experiment begins.
(Description from MD)
A real classic, one of the best romance/comedy manga I've ever read. No status quo, constant development, interesting characters, no real villain.
Originally four volumes long, now apparently re-released in two larger volumes (see the link above). It's been available in French, but for some unfathomable reason there's no sign of an English version anywhere. Scans are a mixed bag with some odd translation choices. This needs to be corrected, IMO.
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I remember reading this, it was a nice series. Not sure it would be much profitable since it's old, but who knows.