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Since when does Vanessa Hudgens have a twin?

The former High School Musical star hit an LA Fitness gym with her sister Stella on Monday afternoon, and it was hard not to do a double take -- not only has V's little sib grown up fast, the two look nearly identical now!

Crazy!

Vanessa has been hitting the gym hard lately -- she had to gain weight for her gritty role in Gimme Shelter, and it looks like she's ready to get back into fighting shape.

Work it, girls!