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Alison doesn't have a -son ending, it has an -on ending, it was a medieval nn for Alice (http://www.s-gabriel.org/names/talan/reaney/reaney.cgi?Alison). Endings like -on and -ot were often used to create pet forms, like Marion for Mary, Magot for Margaret, Belot for Isabel.

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I think Alison / Allison is actually a rare case of a girls' name being used for as a boys as it wasn't originally a boys' name.