Took 400mg of presc magnesium this am before starting colonoscopy prep. Noticed suprep has magnesium. Is it ok that I took magnesium supplement.

Probably ok. The magnesium supplement you took was probably eliminated during the bowel cleansing process, which uses a magnesium-based laxative such as magnesium citrate. The magnesium in the bowel prep isn't absorbed by the body but passes through the intestines and causes the bowel-cleansing cathartic effect. Check with your pcp or gastroenterologist.