Someone at my church has had one. They don't want to take hers out because of the risk of excessive bleeding so they've just added one it and attached it to her leg veins. She also said she knows someone who has 6.
Well, that makes sense, but that's a non-standard case of a patient with coagulation disorder or fragile blood vessels.
I knew the new kidneys are usually placed into the pelvic region - it's easy to hook them up to the communal iliac arteries and they are better accessible for future examinations. It just never occured to me that the old ones would be kept in place. Some diseased kidneys can still produce hormones (EPO, Vit D, renin), which could lead to polycythemia, osteoporosis and hypertension with oedemas (renal hypertension). And if they don't, well, I guess there's no harm in leaving them in the patient, other than ascendent infections of the urether due to lack of urine formation...
Hm. I guess it boils down to weighting up the risks of removal (bleeding, I suppose) and risks of leaving the kidney in.
Anyway, nice to have some food for thought