Toaru Ryoushu no Shuuchakuai no Hate

"You are the savior of this world."

The protagonist Maika is summoned to another world by a young lord named Ash. A world where alchemy is a considered a forbidden art.

As they spend time together, the two naturally become closer, and eventually start a physical relationship... but Maika experiences flashbacks of memories she has no recollection of experiencing.

While Ash is away, she happens upon a warehouse... "Have I been here... before?"
and uncovers a hidden underground laboratory where she finds... Herself. A row of water tanks, with humanoids floating inside... humanoids who look just like her.

From that moment forth, her sweet dreamlike life with the lord... is shattered.