Does a relation between a set and itself require the two elements of each pair to be the same?

"Note that just because a relation’s two sets are the same, that doesn’t necessarily imply that the two elements are the same for any of its ordered pairs. Harry clearly doesn’t have a crush on himself, nor does anyone else have a self-crush. And no soda has more calories than itself, either — that’s impossible. That being said, though, an ordered pair can have the same two elements. Consider the relation “hasSeen” between X and X. Surely all three wizards have looked in a mirror at some point in their lives, so in addition to ordered pairs like (Ron, Harry) the hasSeen relation also contains ordered pairs like (Ron, Ron) and (Hermione, Hermione)."

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