Who had sex with Leah by mistake?

Jacob

And Jacob said unto Laban, Give me my wife, for my days are fulfilled, that I may go in unto her.

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And it came to pass in the evening, that he took Leah his daughter, and brought her to him; and he went in unto her.

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And it came to pass, that in the morning, behold, it was Leah. Genesis 29:23-25