I just discovered the "Challenges" forum today; I must have been blind!
Well anyway, I'm big on brain teasers and exchange them with friends all the time. Here's a good one: (pardon me if it's ever been used before, I'm new here

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There were once two girls who had the same mother and father, were the same height, had the same birthday, and looked identical from head to toe. However, they were not twins. What were they?