Actually, I see it from the opposite point of view, and I think it more likely that two squibs would know each other and maybe marry. Like, if you have a rare disease, you would find yourself in the position to find someone else with that disease, but if you didn't, you wouldn't bother searching for someone with it. Doesn't make a lot of sense once I put it in words. I just think it's more likely that two squibs would find each other.
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And I still think Mrs. Figg and Filch are brother and sister!
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