How rare is it for someone to have double jointed fingers without marfan's or eds?

Not rare at all: Flexibility of the joints, both large and small, is common. Many normal folks have this, and so do folks with certain genetic conditions, like eds and Marfan syndrome, two of hundreds of genetic conditions that feature joint hyperextensibility. Are you worried about these conditions? Bring your concerns up with your regular doctor at your next check up.