How rare is it for someone to have double jointed fingers unless marphans?

Many reasons: Hypermobility generally results from one or more of the following:misaligned joints, abnormally shaped ends of one or more bones at a joint , type 1 collagen or other connective tissue defect (found in ehlers-danlos syndrome, abnormal joint proprioception (an impaired ability to determine where in space parts of the body are, and how stretched a joint.

Marfans is linked to: joint instabilty but there are many similar diseases yhat lead to joint instability. Many younger ( non marfanoid) have joint looseness that they also grow out of.