Bob is attracted to Alice. By some process similar to Oven Logic, he imagines that two Alices must be even sexier than one. Sometimes Bob is just fantasizing, but often there's an actual second "Alice" floating around. This may be a twin (of either gender), a product of Applied Phlebotinum, or more rarely a sibling, parent, cousin, or even just someone unrelated who dresses and acts like Alice. For the record, suggesting this in Real Life is not a good idea. It's even worse to bring up if the person in question does have a twin. Examples of Twin Threesome Fantasy include: