Doubling gravity would put your body under a great deal of stress and could be fatal if you were exposed to it for a protracted period.
Firstly, your joints and skeleton would strain to support your increased weight. Your muscles would struggle with even small movements. However, one of the most dangerous effects would be on your heart. Stronger gravity would cause blood to pool in your extremities, making your heart work extra hard to keep the blood pumping around your body.
Your blood pressure would have to double to overcome the increased gravity. Other processes inside the body might also be affected. Over time, very fit individuals might adapt and build stronger bones and muscles, but any injury or illness would be difficult to recover from.
Falls would be particularly dangerous. People who were less git, overweight or affected by other health conditions would be unlikely to survive a long exposure to double gravity conditions.