I don’t think there is any mistake. Regina obviously just needed a little reminder from Rumple about the curse, that’s all. Of course she knew about it already, because Rumple didn’t have to say much before she realized what he was talking about.
Why she got the Dark Curse from him in the first place, we might now know, it might be that she didn’t even intend to get it but that Rumple sneaked it in somehow. She considered it useless enough for her to trade it with Maleficient – and obviously once trade is done, she would be very unlikely to get it back, thus it wouldn’t be important for her to remember and she simply forgot about it.
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